Picture decoding device, picture decoding method and picture decoding program

ABSTRACT

In a picture coding device for coding picture signals including a brightness signal and a color-difference signal in a block unit using intra-prediction and coding information regarding an intra-prediction mode, when aspect ratios of pixels of the brightness signal and the color-difference signal are different from each other, a bitstream generator converts a mode number of a first intra-color-difference prediction mode used when the aspect ratios are equal to each other into a scaled mode number and derives a second intra-color-difference prediction mode used when the aspect ratios are different from each other.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.14/860,292, filed Sep. 21, 2015, which is a Continuation ofInternational Application No. PCT/JP2014/001510, filed on Mar. 17, 2014,which in turn claims the benefit of Japanese Application No.2013-074913, filed on Mar. 29, 2013, Japanese Application No.2013-074914, filed on Mar. 29, 2013, Japanese Application No.2013-081796, filed on Apr. 10, 2013, Japanese Application No.2013-081797, filed on Apr. 10, 2013, Japanese Application No.2014-023251, filed on Feb. 10, 2014, and Japanese Application No.2014-023252, filed on Feb. 10, 2014, the disclosures of whichApplications are incorporated by reference herein.

BACKGROUND 1. Field of the Invention

The present invention relates to technology for coding and decoding apicture, and particularly, technology for coding and decoding in ascreen.

2. Description of the Related Art

As a representative example of a compression coding system of movingpictures, the standard of MPEG-4 AVC/H.264 is known. In MPEG-4AVC/H.264, coding is performed in units of macro blocks obtained bydividing a picture into a plurality of rectangular blocks. A size of themacro block is defined as 16×16 pixels in a brightness signal,regardless of a picture size. A color-difference signal is also includedin the macro block. However, a size of the color-difference signalincluded in the macro block is different according to a color-differenceformat of a coded picture. When the color-difference format is 4:2:0,the size of the color different signal is defined as 8×8 pixels, whenthe color-difference format is 4:2:2, the size of the color differentsignal is defined as 8×16 pixels, and when the color-difference formatis 4:4:4, the size of the color different signal is defined as 16×16pixels.

In the color-difference format, a ratio of sampled pixel numbers ofthree signals of one piece of brightness information and two pieces ofcolor-difference information is represented by X:Y:Z. As examples of acolor-difference format of a picture to be coded and decoded by MPEG-4AVC/H.264, 4:2:0, 4:2:2, 4:4:4, and a monochrome are known.

FIGS. 3A to 3E are a diagram illustrating each color-difference formatof a picture. x shows a position of a pixel of a brightness signal of apicture on a screen plane and ∘ shows a position of a pixel of acolor-difference signal.

4:2:0 illustrated in FIG. 3A is a color-difference format in which acolor-difference signal is sampled at a density of ½ in both horizontaland vertical directions, with respect to a brightness signal. That is,in 4:2:0, aspect ratios of pixels of the brightness signal and thecolor-difference signal are equal to each other. In 4:2:0, thecolor-difference signal may be sampled at a position illustrated in FIG.3E.

4:2:2 illustrated in FIG. 3B is a color-difference format in which acolor-difference signal is sampled at a density of ½ in a horizontaldirection and at the same density in a vertical direction, with respectto a brightness signal. That is, in 4:2:2, aspect ratios of pixels ofthe brightness signal and the color-difference signal are different fromeach other.

4:4:4 illustrated in FIG. 3C is a color-difference format in which botha brightness signal and a color-difference signal are sampled at thesame density. That is, in 4:4:4, aspect ratios of pixels of thebrightness signal and the color-difference signal are equal to eachother.

The monochrome illustrated in FIG. 3D is a color-difference formatconfigured using only a brightness signal without using acolor-difference signal.

The brightness signal and the color-difference signal are set to sharecoding information such as motion compensation with each other and arecoded and decoded. However, in 4:4:4, a mechanism for coding anddecoding one brightness signal and two color-difference signals as threemonochromes independently is also prepared.

In a system of AVC/H.264, a method of executing prediction from a blockcoded/decoded in a coding/decoding target picture is used. This methodis called intra-prediction. In addition, motion compensation to predicta motion from a reference picture using a coded/decoded picture as thereference picture is used. A method of predicting the motion by themotion compensation is called inter-prediction.

First, switching units of an intra-prediction mode by theintra-prediction in intra-coding of the system of AVC/H.264 will bedescribed. FIGS. 4A to 4C are diagrams illustrating the switching unitsof the intra-prediction mode. In the intra-coding of the system ofAVC/H.264, three types of “4×4 intra-prediction”, “16×16intra-prediction”, and “8×8 intra-prediction” are prepared as theswitching units of the intra-prediction mode.

In the “4×4 intra-prediction”, a brightness signal of a macro block (abrightness signal of 16×16 pixel blocks and a color-difference signal of8×8 pixel blocks) is divided into 16 parts of 4×4 pixel blocks, a modeis selected from nine types of 4×4 intra-prediction modes in 4×4 pixelunits divided, and the intra-prediction is sequentially performed (FIG.4A).

In the “16×16 pixel intra-prediction”, a mode is selected from fourtypes of 16×16 intra-prediction modes in 16×16 pixel block units of abrightness signal and the intra-prediction is performed (FIG. 4B).

In the “8×8 pixel intra-prediction”, a brightness signal of a macroblock is divided into 4 parts of 8×8 pixel blocks, a mode is selectedfrom nine types of 8×8 intra-prediction modes in 8×8 pixel unitsdivided, and the intra-prediction is sequentially performed (FIG. 4C).

In addition, in the intra-prediction of the color-difference signal,when the color-difference format is 4:2:0 or 4:2:2, a mode is selectedfrom four types of intra-prediction modes of the color-difference signalin macro block units and the intra-predictions are performed.

Next, units of the inter-prediction in inter coding of the system ofAVC/H.264 will be described. FIGS. 5A to 5H are diagrams illustratingmacro block partition and sub-macro block partition. Here, only a pixelblock of a brightness signal is illustrated for the simplification ofexplanation. In MPEG series, a macro block is defined by a squareregion. Generally, in the MPEG series including the system of AVC/H.264,a block defined by 16×16 pixels (16 pixels in a horizontal direction and16 pixels in a vertical direction) is called a macro block. In addition,in the system of AVC/H.264, a block defined by 8×8 pixels is called asub-macro block. The macro block partition means each of small blocksobtained by dividing the macro block for the sake of motion compensationand prediction. The sub-macro block partition means each of small blocksobtained by dividing the sub-macro block for the sake of the motioncompensation and prediction.

FIG. 5A is a diagram illustrating the case in which a macro block isconfigured from one macro block partition configured from a brightnesssignal of 16×16 pixels and two color-difference signals corresponding tothe brightness signal. Here, this configuration is called a macro blocktype of a 16×16 mode.

FIG. 5B is a diagram illustrating the case in which a macro block isconfigured from two macro block partitions configured from a brightnesssignal of 16×8 pixels (16 pixels in a horizontal direction and 8 pixelsin a vertical direction) and two color-difference signals correspondingto the brightness signal. The two macro block partitions are arrangedvertically. Here, this configuration is called a macro block type of a16×8 mode.

FIG. 5C is a diagram illustrating the case in which a macro block isconfigured from two macro block partitions configured from a brightnesssignal of 8×16 pixels (8 pixels in a horizontal direction and 16 pixelsin a vertical direction) and two color-difference signals correspondingto the brightness signal. The two macro block partitions are arrangedhorizontally. Here, this configuration is called a macro block type ofan 8×16 mode.

FIG. 5D is a diagram illustrating the case in which a macro block isconfigured from four macro block partitions configured from a brightnesssignal of 8×8 pixels and two color-difference signals corresponding tothe brightness signal. The four macro block partitions are arranged twoby two vertically and horizontally. Here, this configuration is called amacro block type of an 8×8 mode.

FIG. 5E is a diagram illustrating the case in which a sub-macro block isconfigured from one sub-macro block partition configured from abrightness signal of 8×8 pixels and two color-difference signalscorresponding to the brightness signal. Here, this configuration iscalled a sub-macro block type of an 8×8 mode.

FIG. 5F is a diagram illustrating the case in which a sub-macro block isconfigured from two sub-macro block partitions configured from abrightness signal of 8×4 pixels (8 pixels in a horizontal direction and4 pixels in a vertical direction) and two color-difference signalscorresponding to the brightness signal. The two sub-macro blockpartitions are arranged vertically. This configuration is called asub-macro block type of an 8×4 mode.

FIG. 5G is a diagram illustrating the case in which a sub-macro block isconfigured from two macro block partitions configured from a brightnesssignal of 4×8 pixels (4 pixels in a horizontal direction and 8 pixels ina vertical direction) and two color-difference signals corresponding tothe brightness signal. The two macro block partitions are arrangedhorizontally. Here, this configuration is called a sub-macro block typeof a 4×8 mode.

FIG. 5H is a diagram illustrating the case in which a sub-macro block isconfigured from four sub-macro block partitions configured from abrightness signal of 4×4 pixels and two color-difference signalscorresponding to the brightness signal. The four sub-macro blockpartitions are arranged two by two vertically and horizontally. Here,this configuration is called a sub-macro block type of a 4×4 mode.

In the coding system of AVC/H.264, a mechanism for selectively using themotion compensation block sizes is taken. First, any macro block typecan be selected as the motion compensation block size of the macro blockunit, from the macro block types of the 16×16, 16×8, 8×16, and 8×8modes. When the macro block type of the 8×8 mode is selected, anysub-macro block type can be selected as the motion compensation blocksize of the sub-macro block unit, from the sub-macro block types of the8×8, 8×4, 4×8, and 4×4 modes.

CITATION LIST Non-Patent Literature

Non-Patent Literature 1: ISO/IEC14496-10 Information technology—Codingof audio-visual objects—Part 10: Advanced Video Coding

When information regarding an intra-prediction mode of a picture signalis coded, information regarding an intra-prediction mode of a brightnesssignal and information regarding an intra-prediction mode of acolor-difference signal are coded and are arranged in a bitstream.However, at this time, if the intra-prediction mode is not codedaccording to a color-difference format, process efficiency may bedegraded.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand an object of the present invention is to provide technology forefficiently coding and decoding a picture signal by intra-prediction ofa brightness signal and a color-difference signal according to acolor-difference format.

In order to solve the above issue, a picture coding device of an aspectof the present invention for coding information regarding anintra-prediction mode in a prediction block unit and coding picturesignals including a brightness signal and a color-difference signal in aconversion block unit using intra-prediction, includes: anintra-brightness prediction mode coder (122, 126) that sets a predictionblock of the brightness signal, codes a syntax element regarding theintra-prediction mode of the brightness signal, on the basis of theintra-prediction mode of the brightness signal showing anintra-prediction method of the prediction block of the bright signal,and codes information regarding an intra-brightness prediction mode in abitstream; an intra-color-difference prediction mode coder (123, 126)that sets a prediction block of the color-difference signal, codes asyntax element regarding an intra-color-difference prediction mode ofthe color-difference signal by referring to the intra-brightnessprediction mode, on the basis of the intra-color-difference predictionmode of the color-difference signal showing an intra-prediction methodof the prediction block of the color-difference signal, and codesinformation regarding the intra-color-difference prediction mode in thebitstream; a brightness signal intra-predictor (103) that predicts abrightness signal of a conversion block of the brightness signal from asurrounding brightness signal of the conversion block of the brightnesssignal, according to the intra-brightness prediction mode; and acolor-difference signal intra-predictor (103) that predicts acolor-difference signal of a conversion block of the color-differencesignal from a surrounding color-difference signal of the conversionblock of the color-difference signal, according to theintra-color-difference prediction mode.

According to another aspect of the present invention, there is provideda picture coding method. The picture coding method for codinginformation regarding an intra-prediction mode in a prediction blockunit and coding picture signals including a brightness signal and acolor-difference signal in a conversion block unit usingintra-prediction, includes: setting a prediction block of the brightnesssignal, coding a syntax element regarding the intra-prediction mode ofthe brightness signal, on the basis of the intra-prediction mode of thebrightness signal showing an intra-prediction method of the predictionblock of the bright signal, and coding information regarding anintra-brightness prediction mode in a bitstream; setting a predictionblock of the color-difference signal, coding a syntax element regardingan intra-color-difference prediction mode of the color-difference signalby referring to the intra-brightness prediction mode, on the basis ofthe intra-color-difference prediction mode of the color-differencesignal showing an intra-prediction method of the prediction block of thecolor-difference signal, and coding information regarding theintra-color-difference prediction mode in the bitstream; predicting abrightness signal of a conversion block of the brightness signal from asurrounding brightness signal of the conversion block of the brightnesssignal, according to the intra-brightness prediction mode; andpredicting a color-difference signal of a conversion block of thecolor-difference signal from a surrounding color-difference signal ofthe conversion block of the color-difference signal, according to theintra-color-difference prediction mode.

According to further another aspect of the present invention there isprovided a picture decoding device. The picture decoding device fordecoding information regarding an intra-prediction mode in a predictionblock unit and decoding picture signals including a brightness signaland a color-difference signal in a conversion block unit usingintra-prediction, includes: an intra-brightness prediction mode decoder(222, 224) that decodes a syntax element regarding an intra-predictionmode of the brightness signal from a bitstream in which informationregarding an intra-brightness prediction mode showing anintra-prediction method of a prediction block of the bright signal iscoded and derives the intra-prediction mode of the brightness signal; anintra-color-difference prediction mode decoder (222, 225) that decodes asyntax element regarding an intra-color-difference prediction mode ofthe color difference signal from the bitstream in which informationregarding an intra-color-difference prediction mode showing anintra-prediction method of a prediction block of the color-differencesignal is coded and derives the intra-color-difference prediction modeby referring to the intra-brightness prediction mode; a brightnesssignal intra-predictor (206) that predicts a brightness signal of aconversion block of the brightness signal from a surrounding brightnesssignal of the conversion block of the brightness signal, according tothe intra-brightness prediction mode specified for each prediction blockof the brightness signal; and a color-difference signal intra-predictor(206) that predicts a color-difference signal of a conversion block ofthe color-difference signal from a surrounding color-difference signalof the conversion block of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal.

According to further another aspect of the present invention there isprovided a picture decoding method. The picture decoding method fordecoding information regarding an intra-prediction mode in a predictionblock unit and decoding picture signals including a brightness signaland a color-difference signal in a conversion block unit usingintra-prediction, includes: decoding a syntax element regarding anintra-prediction mode of the brightness signal from a bitstream in whichinformation regarding an intra-brightness prediction mode showing anintra-prediction method of a prediction block of the bright signal iscoded and deriving the intra-prediction mode of the brightness signal;decoding a syntax element regarding an intra-color-difference predictionmode of the color-difference signal from the bitstream in whichinformation regarding an intra-color-difference prediction mode showingan intra-prediction method of a prediction block of the color-differencesignal is coded and deriving the intra-color-difference prediction modeby referring to the intra-brightness prediction mode; predicting abrightness signal of a conversion block of the brightness signal from asurrounding brightness signal of the conversion block of the brightnesssignal, according to the intra-brightness prediction mode specified foreach prediction block of the brightness signal; and predicting acolor-difference signal of a conversion block of the color-differencesignal from a surrounding color-difference signal of the conversionblock of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal.

In addition, any combinations of the above components and conversionexpressions of the present invention among a method, a device, a system,a recording medium, and a computer program are effective as aspects ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a picturecoding device according to an embodiment;

FIG. 2 is a block diagram illustrating a configuration of a picturedecoding device according to an embodiment;

FIGS. 3A to 3E are a diagram illustrating a color-difference format of apicture;

FIGS. 4A to 4C are a diagram illustrating switching units of anintra-prediction mode of a system of AVC/H.264;

FIGS. 5A to 5H are a diagram illustrating units of inter-prediction ofthe system of AVC/H.264;

FIG. 6 is a diagram illustrating a tree block and a coding block definedin this embodiment;

FIGS. 7A to 7D are a diagram illustrating a division mode defined inthis embodiment;

FIG. 8 is a diagram illustrating a value and a prediction direction ofan intra-prediction mode defined in this embodiment;

FIG. 9 is an exemplary diagram illustrating a position of a blockdefined in this embodiment;

FIG. 10 is a diagram illustrating an example of a definition of a syntaxwhen color-difference format information is coded by a sequenceparameter set becoming a header to code information regarding coding ofan entire sequence defined in this embodiment;

FIGS. 11A to 11D is a diagram illustrating a division method of acolor-difference signal of a coding block in N×N division at the time ofintra-prediction defined in this embodiment;

FIG. 12 is a block diagram illustrating a configuration of a secondbitstream generator of the picture coding device according to theembodiment;

FIG. 13 is a block diagram illustrating a configuration of a secondbitstream decoder of the picture decoding device according to theembodiment;

FIG. 14 is a table to derive a value of an intra-color-differenceprediction mode from a value of a syntax element used at a decoding sidedefined in this embodiment and a value of an intra-brightness predictionmode of a prediction block of a brightness signal of the same positionas a prediction block of a color-difference signal;

FIG. 15 is a diagram illustrating an example of a conversion table toconvert an intra-brightness prediction mode or a firstintra-color-difference prediction mode into a secondintra-color-difference prediction mode at a color-difference format4:2:2 defined in this embodiment;

FIG. 16 is a diagram illustrating a conversion table to convert anintra-brightness prediction mode or a first intra-color-differenceprediction mode into a second intra-color-difference prediction mode ata color-difference format 4:2:2 defined in this embodiment;

FIG. 17 is a diagram illustrating another example of a conversion tableto convert an intra-brightness prediction mode or a firstintra-color-difference prediction mode into a secondintra-color-difference prediction mode at a color-difference format4:2:2 defined in this embodiment;

FIG. 18 is a diagram illustrating a value and a prediction direction ofan intra-prediction mode of the conversion table of FIG. 15 defined inthis embodiment;

FIG. 19 is a diagram illustrating a value and a prediction direction ofan intra-prediction mode of the conversion table of FIG. 16 defined inthis embodiment;

FIG. 20 is a diagram illustrating a value and a prediction direction ofan intra-prediction mode of the conversion table of FIG. 17 defined inthis embodiment;

FIGS. 21A and 21B are a diagram illustrating a correspondence relationof prediction directions of intra-predictions of a brightness signal anda color-difference signal when a color-difference format is 4:2:2;

FIGS. 22A and 22B are a diagram illustrating a correspondence relationof prediction directions of intra-predictions of a brightness signal anda color-difference signal when a color-difference format is 4:2:0;

FIG. 23 is a diagram illustrating a derivation process sequence when anintra-brightness prediction mode or a first intra-color-differenceprediction mode is converted into a second intra-color-differenceprediction mode, corresponding to the conversion tables of FIGS. 15 and16 defined in this embodiment;

FIG. 24 is a diagram illustrating a derivation process sequence when afirst intra-color-difference prediction mode is converted into a secondintra-color-difference prediction mode, corresponding to the conversiontable of FIG. 17 defined in this embodiment;

FIG. 25 is a table to derive a value of a syntax element regarding anintra-color-difference prediction mode from a value of anintra-color-difference prediction mode used at a coding side defined inthis embodiment and a value of an intra-brightness prediction mode of aprediction block of a brightness signal of the same position as aprediction block of a color-difference signal;

FIG. 26 is a diagram illustrating a process sequence of decoding of anintra-prediction mode and intra-prediction of a decoding side;

FIG. 27 is a diagram illustrating an example of a table to derive anangle of intra-prediction for a color-difference format 4:2:2 from afirst intra-color-difference prediction mode;

FIG. 28 is a diagram illustrating another example of a table to derivean angle of intra-prediction for a color-difference format 4:2:2 from afirst intra-color-difference prediction mode;

FIG. 29 is a diagram illustrating a derivation process sequence toderive an angle of intra-prediction for a color-difference format 4:2:2from a first intra-color-difference prediction mode, corresponding tothe table of FIG. 28 defined in this embodiment;

FIG. 30 is a diagram illustrating a conversion table to convert anintra-brightness prediction mode or a first intra-color-differenceprediction mode into a second intra-color-difference prediction mode ata color-difference format 4:2:2 defined in this embodiment;

FIG. 31 is a diagram illustrating a value and a prediction direction ofan intra-prediction mode of the conversion table of FIG. 30 defined inthis embodiment;

FIG. 32 is a diagram illustrating a derivation process sequence when afirst intra-color-difference prediction mode is converted into a secondintra-color-difference prediction mode, corresponding to the conversiontable of FIG. 30 defined in this embodiment;

FIG. 33 is a diagram illustrating a conversion table to convert anintra-brightness prediction mode or a first intra-color-differenceprediction mode into a second intra-color-difference prediction mode ata color-difference format 4:2:2 defined in this embodiment;

FIG. 34 is a diagram illustrating a value and a prediction direction ofan intra-prediction mode of the conversion table of FIG. 33 defined inthis embodiment; and

FIG. 35 is a diagram illustrating a derivation process sequence when afirst intra-color-difference prediction mode is converted into a secondintra-color-difference prediction mode, corresponding to the conversiontable of FIG. 33 defined in this embodiment.

DETAILED DESCRIPTION

The invention will now be described by reference to the preferredembodiments. This does not intend to limit the scope of the presentinvention, but to exemplify the invention.

This embodiment relates to coding and decoding of moving pictures.Particularly, in this embodiment, a code amount is reduced usingintra-prediction to perform prediction from pixel values of surroundingcoded and decoded blocks in coding and pixel values of neighboringdecoded blocks in decoding and inter-prediction by motion compensationfrom a decoded picture, in units of blocks obtained by dividing apicture into rectangles having any size and shape.

First, technologies and technical terms used in this embodiment aredefined.

(Color-Difference Format)

In description of an embodiment, it is assumed that color-differenceformats of a picture to be coded and decoded are set to a monochrome,4:2:0, 4:2:2, and 4:4:4 and a brightness signal and a color-differencesignal are set and are coded and decoded. However, in description of thecolor-difference signal, description of the case of the monochrome isomitted. When the color-difference format is 4:4:4, RGB signals can becoded and decoded. In this case, a G (green) signal is regarded as abrightness signal, a B (blue) signal and an R (red) signal are regardedas color-difference signals, and the signals are coded and decoded. Amethod of coding and decoding the brightness signal and thecolor-difference signals independently at 4:4:4 is regarded as themonochrome in this embodiment.

(With Respect to Tree Block and Coding Block)

In the embodiment, as illustrated in FIG. 6, a screen is equally dividedin units of squares having any same size. This unit is defined as a treeblock and is used as a basic unit of address management to specify acoding/decoding target block (a coding target block in coding and adecoding target block in decoding) in a picture. The tree block otherthan the monochrome is configured from one brightness signal and twocolor-difference signals. A size of the tree block can be freely set insizes of a power of 2, according to a picture size or a texture in thescreen. To optimize a coding process according to the texture in thescreen, the tree block can be changed to blocks having a small blocksize, by dividing brightness signals and color-difference signals in thetree block by 4 (two division in each of a horizontal direction and avertical direction) hierarchically, according to necessity. Each ofthese blocks is defined as a coding block and is used as a basic unit ofa process when coding and decoding are performed. The coding block otherthan the monochrome is also configured from one brightness signal andtwo color-difference signals. A maximum size of the coding block isequal to the size of the tree block. A coding block having a minimumsize of the coding blocks is called a minimum coding block and the sizethereof can be freely set in sizes of a power of 2.

In FIG. 6, a coding block A is a block of the case in which the treeblock is not divided and is used as one coding block. Coding blocks Bare coding blocks obtained by dividing the tree block into four blocks.Coding blocks C are coding blocks obtained by dividing each of blocksobtained by dividing the tree block into four blocks into four blocks.Coding blocks Dare coding blocks obtained by dividing each of blocksobtained by dividing the tree block into four blocks into four blockstwo times hierarchically and are coding blocks having a minimum size.

In description of the embodiment, when the color-difference format is4:2:0, the size of the tree block is set to 64×64 pixels in thebrightness signal and 32×32 pixels in the color-difference signal andthe size of the minimum coding block is set to 8×8 pixels in thebrightness signal and 4×4 pixels in the color-difference signal. In FIG.6, the size of the coding block A is 64×64 pixels in the brightnesssignal and 32×32 pixels in the color-difference signal, the size of thecoding block B is 32×32 pixels in the brightness signal and 16×16 pixelsin the color-difference signal, the size of the coding block C is 16×16pixels in the brightness signal and 8×8 pixels in the color-differencesignal, and the size of the coding block D is 8×8 pixels in thebrightness signal and 4×4 pixels in the color-difference signal. Whenthe color-difference format is 4:4:4, the sizes of the brightness signaland the color-difference signal of each coding block are equal to eachother. When the color-difference format is 4:2:2, the size of the codingblock A is 32×64 pixels in the color-difference signal, the size of thecoding block B is 16×32 pixels in the color-difference signal, the sizeof the coding block C is 8×16 pixels in the color-difference signal, andthe size of the coding block D to be the minimum coding block is 4×8pixels in the color-difference signal.

(With Respect to Prediction Mode)

The intra-prediction to perform prediction from surroundingcoded/decoded picture signals and the inter-prediction to performprediction from picture signals of coded/decoded pictures are switchedin a coding block unit. A mode to identify the intra-prediction and theinter-prediction is defined as a prediction mode (PredMode). Theprediction mode (PredMode) has a value of intra-prediction (MODE_INTRA)or inter-prediction (MODE_INTER) and can be selected and coded.

(With Respect to Division Mode and Prediction Block)

When the screen is divided into blocks and the intra-prediction and theinter-prediction are performed, a coding block is divided according tonecessity to cause switching units of methods of the intra-predictionand the inter-prediction to be smaller and the prediction is performed.A mode to identify a division method of the brightness signal and thecolor-difference signal of the coding block is defined as a divisionmode (PartMode). The divided block is defined as a prediction block. Asillustrated in FIGS. 7A to 7D, four types of division modes (PartMode)are defined according to the division method of the brightness signal ofthe coding block. A division mode (PartMode) of the case (FIG. 7A) inwhich the brightness signal of the coding block is not divided and thecoding block is regarded as one prediction block is defined as 2N×2Ndivision (PART_2N×2N), a division mode (PartMode) of the case (FIG. 7B)in which the brightness signal of the coding block is divided into twoblocks in a horizontal direction and the coding block is used as twoprediction blocks is defined as 2N×N division (PART_2N×N), a divisionmode (PartMode) of the case (FIG. 7C) in which the brightness signal ofthe coding block is divided in a vertical direction and the coding blockis used as two prediction blocks is defined as N×2N division(PART_N×2N), and a division mode (PartMode) of the case (FIG. 7D) inwhich the brightness signal of the coding block is divided equally in ahorizontal direction and a vertical direction and the coding block isused as four prediction blocks is defined as N×N division (PART_N×N).The color-difference signal is also divided at the same division ratioas the division ratio of the vertical and horizontal directions of thebrightness signal, for each division mode (PartMode) other than the N×Ndivision (PART_N×N) of the intra-prediction (MODE_INTRA). The divisionratio of the vertical and horizontal directions of the color-differencesignal of the coding block of the N×N division (PART_N×N) of theintra-prediction (MODE_INTRA) is different according to the type of thecolor-difference format, which will be described below.

A number starting from 0 is allocated to each of the prediction blocksexisting in the coding block in coding order to specify each predictionblock in the coding block. The number is defined as a division indexPartIdx. A numeral described in each prediction block of the codingblock of FIGS. 7A to 7D shows a division index PartIdx of eachprediction block. In the 2N×N division (PART_2N×N) illustrated in FIG.7B, a division index PartIdx of an upper prediction block is defined as0 and a division index PartIdx of a lower prediction block is definedas 1. In the N×2N division (PART_N×2N) illustrated in FIG. 7C, adivision index PartIdx of a left prediction block is defined as 0 and adivision index PartIdx of a right prediction block is defined as 1. Inthe N×N division (PART_N×N) illustrated in FIG. 7D, a division indexPartIdx of an upper left prediction block is defined as 0, a divisionindex PartIdx of an upper right prediction block is defined as 1, adivision index PartIdx of a lower left prediction block is defined as 2,and a division index PartIdx of a lower right prediction block isdefined as 3.

When the prediction mode (PredMode) is the intra-prediction(MODE_INTRA), the division mode (PartMode) is defined as the 2N×2Ndivision (PART_2N×2N) in the coding blocks other than the coding block D(in this embodiment, 8×8 pixels in the brightness signal) to be theminimum coding block and the division mode (PartMode) is defined as the2N×2N division (PART_2N×2N) and the N×N division (PART_N×N) in only thecoding block D to be the minimum coding block.

When the prediction mode (PredMode) is the inter-prediction(MODE_INTER), the division mode (PartMode) is defined as the 2N×2Ndivision (PART_2N×2N), the 2N×N division (PART_2N×N), and the N×2Ndivision (PART_N×2N) in the coding blocks other than the coding block Dto be the minimum coding block and the division mode (PartMode) isdefined as the 2N×2N division (PART_2N×2N), the 2N×N division(PART_2N×N), the N×2N division (PART_N×2N), and the N×N division(PART_N×N) in only the coding block D to be the minimum coding block.The reason why the N×N division (PART_N×N) is not defined in the codingblocks other than the minimum coding block is that the coding block canbe divided into four blocks and the small coding block can berepresented, in the coding blocks other than the minimum coding block.

(With Respect to Intra-Prediction and Intra-Prediction Mode)

In the intra-prediction, values of pixels of a process target conversionblock are predicted from values of pixels of a surrounding decodedconversion block to be described below in the same screen. In a codingdevice and a decoding device according to this embodiment, anintra-prediction mode is selected from 35 intra-prediction modes foreach prediction block and intra-prediction is performed for eachconversion block. Sizes of the prediction block and the conversion blockmay be different from each other. However, when the intra-prediction ofthe conversion block is performed, an intra-prediction mode of aprediction block including the conversion block is used. FIG. 8 is adiagram illustrating a value and a prediction direction of anintra-prediction mode defined in this embodiment. The value of theintra-prediction mode is defined by mode numbers from 0 to 34. Theintra-prediction mode (intraPredMode) defines plane prediction(intra-prediction mode intraPredMode=0) in which prediction is performedby interpolating a pixel value from a surrounding decoded block, averagevalue prediction (intra-prediction mode intraPredMode=1) in whichprediction is performed by deriving an average value from a surroundingdecoded block, and 33 angle predictions (intra-prediction modeintraPredMode=2 . . . 34) in which prediction is performed from asurrounding decoded block at various angles.

(Conversion Block)

Similarly to the conventional technology, in this embodiment, a codeamount is reduced using orthogonal transform to convert a discretesignal into a frequency domain, such as discrete cosine transform (DCT)and discrete sine transform (DST), and inverse transform thereof.Conversion or inverse conversion is performed in units of conversionblocks obtained by dividing a coding block into four partshierarchically. In the embodiment, four conversion sizes of 32×32pixels, 16×16 pixels, 8×8 pixels, and 4×4 pixels are defined and 32×32conversion, 16×16 conversion, 8×8 conversion, 4×4 conversion, andinverse conversion thereof are performed.

(Positions of Tree Block, Coding Block, Prediction Block, and ConversionBlock)

For a position of each of the blocks including the tree block, thecoding block, the prediction block, and the conversion block describedin this embodiment, a position of a pixel of a top-left brightnesssignal included in a region of each block is represented by thetwo-dimensional coordinates of (x, y), using a position of a pixel of atop-left brightness signal in a screen of brightness signals as anoriginal point (0, 0). In directions of coordinate axes, a rightwarddirection in a horizontal direction and a downward direction in avertical direction are positive directions and a unit is a unit of onepixel of a brightness signal. In addition to the case of thecolor-difference format 4:4:4 in which picture sizes (pixel numbers) areequal in a brightness signal and a color-difference signal, in the casesof the color-difference formats 4:2:0 and 4:2:2 in which picture sizes(pixel numbers) are different in a brightness signal and acolor-difference signal, a position of each block of thecolor-difference signal is represented by the coordinates of a positionof a pixel of a brightness signal included in a region of each block anda unit is one pixel of the brightness signal. In this way, the positionof each block of the color-difference signal can be specified and aposition relation of a block of the brightness signal and a block of thecolor-difference signal becomes clear by comparing values of thecoordinates. FIG. 9 is an exemplary diagram illustrating a position of ablock defined in this embodiment, when the color-difference format is4:2:0. In FIG. 9, x shows a position of a pixel of a brightness signalof a picture on a screen plane and ∘ shows a position of a pixel of acolor-difference signal. In FIG. 9, a rectangle shown by a dotted lineis a block E of brightness signals of 8×8 pixels and is a block F ofcolor-difference signals of 4×4 pixels. ▴ is a position of a pixel of atop-left brightness signal of the block E of the brightness signals ofthe 8×8 pixels shown by the dotted line. Therefore, ▴ becomes a positionof the block E of the brightness signals of the 8×8 pixels shown by thedotted line and the coordinates of the brightness signal of the pixelshown by ▴ become the coordinates of the block E of the brightnesssignals of the 8×8 pixels shown by the dotted line. Likewise, ▴ is aposition of a pixel of a top-left brightness signal included in a regionof the block F of the color-difference signals of the 4×4 pixels shownby the dotted line. Therefore, ▴ becomes a position of the block F ofthe color difference signals of the 4×4 pixels shown by the dotted lineand the coordinates of the brightness signal of the pixel shown by Abecome the coordinates of the block F of the color difference signals ofthe 4×4 pixels shown by the dotted line. In the embodiment, only whenvalues of x components and y components of the defined coordinates ofthe block of the brightness signal and the defined coordinates of theblock of the color-difference signal are equal to each other, regardlessof the type of the color difference format or the shape and the size ofthe block, these blocks are defined as blocks at the same position.

FIG. 1 is a block diagram illustrating a configuration of a picturecoding device according to the embodiment. The picture coding deviceaccording to the embodiment includes a color-difference format setter101, a picture memory 102, an intra-predictor 103, an inter-predictor104, a coding method determiner 105, a residual signal generator 106, anorthogonal transformer/quantizer 107, an inverse quantizer/inverseorthogonal transformer 108, a decoding picture signal superimposer 109,a decoding picture memory 111, a first bitstream generator 112, a secondbitstream generator 113, a third bitstream generator 114, and abitstream multiplexer 115.

In the color-difference format setter 101, a color-difference format ofa picture signal of a coding target is set. The color-difference formatmay be determined from a coding picture signal supplied to thecolor-difference format setter 101 and the color-difference format maybe set or the color-difference format may be set from the outside.Information of a color-difference format set to 4:2:0, 4:2:2, or 4:4:4in only a brightness signal is supplied to the first bitstream generator112 and the second bitstream generator 113 and a coding process isexecuted on the basis of the color-difference format. Although notillustrated in the drawings, in the picture memory 102, theintra-predictor 103, the inter-predictor 104, the coding methoddeterminer 105, the residual signal generator 106, the orthogonaltransformer/quantizer 107, the inverse quantizer/inverse orthogonaltransformer 108, the decoding picture signal superimposer 109, and thethird bitstream generator 114 of FIG. 1, the coding process is executedon the basis of the set color-difference format and in a codinginformation memory 110 and the decoding picture memory 111, a managementis made on the basis of the set color-difference format.

In the picture memory 102, picture signals of coding targets supplied inorder of time are temporarily stored. The picture signals of the codingtargets stored in the picture memory 102 are arranged in order ofcoding, are divided in each coding block unit by a plurality ofcombinations according to setting, are divided in each prediction blockunit, and are supplied to the intra-predictor 103 and theinter-predictor 104.

The intra-predictor 103 sets a plurality of intra-brightness predictionmodes and a plurality of intra-color-difference prediction modes foreach of a brightness signal and a color-difference signal of aprediction block of a coding target from a decoded picture signal storedin the decoding picture memory 111, in a prediction block unit accordingto each division mode (PartMode) in a plurality of coding block units,performs each intra-prediction according to an intra-brightnessprediction mode and an intra-color-difference prediction mode for eachconversion block, and obtains an intra-prediction signal. Theintra-color-difference prediction mode can select a value predicted fromthe intra-brightness prediction mode or select any one of 0 (planeprediction), 1 (average value prediction), 10 (horizontal prediction),26 (vertical prediction), and 34 (oblique prediction) to berepresentative intra-prediction modes for each prediction block.However, in this embodiment, when the intra-prediction in which thecolor-difference format is 4:2:2 is performed, a secondintra-color-difference prediction mode to be described below is used.The intra-prediction and the intra-prediction mode of thecolor-difference signal will be described in detail below.

An intra-prediction signal of a prediction block unit is subtracted froma signal of a coding target supplied in the prediction block unit foreach pixel and a prediction residual signal is obtained. An evaluationvalue to evaluate a code amount and a distortion amount is derived usingthe prediction residual signal, an optimal mode is selected from theplurality of intra-brightness prediction modes and the plurality ofintra-color-difference prediction modes in the prediction block unit,from the viewpoint of the code amount and the distortion amount, andintra-prediction information, an intra-prediction signal, and anevaluation value of the intra-prediction corresponding to the selectedintra-prediction mode are supplied as candidates of the intra-predictionof the prediction block to the coding method determiner 105.

The inter-predictor 104 performs each inter-prediction according to aplurality of inter-prediction modes (L0 prediction, L1 prediction, andboth predictions) and a reference picture from the decoded picturesignals stored in the decoding picture memory 111, in a unit accordingto each division mode (PartMode) in the plurality of coding block units,that is, a prediction block unit, and obtains an inter-predictionsignal. At this time, a motion vector search is performed and theinter-prediction is performed according to a searched motion vector. Inthe case of both predictions, two inter-prediction signals are averagedor are weighted and added for each pixel and the inter-prediction ofboth predictions is performed. An inter-prediction signal of aprediction block unit is subtracted from a signal of a coding targetsupplied in the prediction block unit for each pixel and a predictionresidual signal is obtained. An evaluation value to evaluate a codeamount and a distortion amount is derived using the prediction residualsignal, an optimal mode is selected from the plurality ofinter-prediction modes in the prediction block unit, from the viewpointof the code amount and the distortion amount, and inter-predictioninformation, an inter-prediction signal, and the evaluation value of theinter-prediction corresponding to the selected inter-prediction mode aresupplied as candidates of the inter-prediction of the prediction blockto the coding method determiner 105.

The coding method determiner 105 determines a coding block divisionmethod, a prediction mode (PredMode), and a division mode (PartMode)optimally, on the basis of the intra-prediction evaluation value and theinter-prediction evaluation value respectively corresponding to theintra-prediction information and the inter-prediction informationselected for each prediction block in the plurality of coding blockunits, supplies coding information including the intra-predictioninformation or the inter-prediction information according to thedetermination to the second bitstream generator 113, stores the codinginformation in the coding information memory 110, and supplies aprediction signal of the intra-prediction or the inter-predictionaccording to the determination to the residual signal generator 106 andthe decoding picture signal superimposer 109.

The residual signal generator 106 subtracts the prediction signal of theintra-prediction or the inter-prediction from a picture signal to becoded for each pixel, generates a residual signal, and supplies theresidual signal to the orthogonal transformer/quantizer 107.

The orthogonal transformer/quantizer 107 performs orthogonal transformand quantization to convert a signal into a frequency domain, such asthe DCT or the DST, for the supplied residual signal, according to aquantization parameter, generates an orthogonally transformed/quantizedresidual signal, and supplies the orthogonally transformed/quantizedresidual signal to the third bitstream generator 114 and the inversequantizer/inverse orthogonal transformer 108.

The first bitstream generator 112 derives a value of a syntax elementregarding coding information of sequence, picture, and slice units,according to a semantics rule defining a meaning of the syntax elementand a derivation method, performs entropy coding using variable lengthcoding and arithmetic coding for the derived value of each syntaxelement, according to a syntax rule, generates a first bitstream, andsupplies the coded first bitstream to the bitstream multiplexer 115. Avalue of a syntax element regarding the color-difference format is alsoderived by the first bitstream generator 112. The syntax elementregarding the color-difference format is derived from thecolor-difference formation information supplied from thecolor-difference format setter 101. FIG. 10 illustrates an example of adefinition of a syntax when color-difference format information is codedby a sequence parameter set becoming a header to code informationregarding coding of an entire sequence defined in this embodiment. Asyntax element chroma_format_idc shows a type of the color-differenceformat. The syntax element chroma_format_idc shows a monochrome when avalue is 0, 4:2:0 when the value is 1, 4:2:2 when the value is 2, and4:4:4 when the value is 3. In addition, a syntax elementseparate_colour_plane_flag shows whether a brightness signal and acolor-difference signal are coded separately and when a value ofseparate_colour_plane_flag is 0, this shows that the twocolor-difference signals are associated with the brightness signal andthe signals are coded. When the value of the syntax elementchroma_format_idc is 1, this shows that the brightness signal and thetwo color-difference signals are coded separately. Only when the valueof the syntax element chroma_format_idc is 3, that is, thecolor-difference format is 4:4:4, the value of chroma_format_idc can beset to 0 or 1. In the other color-difference formats, the value of thesyntax element separate_colour_plane_flag is set to 0 at all times andthe signals are coded.

The second bitstream generator 113 derives a value of a syntax elementregarding the coding information determined by the coding methoddeterminer 105 for each prediction block, in addition to codinginformation of a coding block unit, according to the semantics ruledefining the meaning of the syntax element and the derivation method.Specifically, the second bitstream generator 113 derives a value of asyntax element regarding coding information of a prediction block unit,in addition to the coding information of the coding block unit such asthe coding block division method, the prediction mode (PredMode), andthe division mode (PartMode). When the prediction mode (PredMode) is theintra-prediction, the second bitstream generator 113 derives a value ofa syntax element regarding an intra-prediction mode including anintra-brightness prediction mode and an intra-color-differenceprediction mode and when the prediction mode (PredMode) is theinter-prediction, the second bitstream generator 113 derives a value ofa syntax element regarding the inter-prediction mode, informationspecifying a reference picture, and inter-prediction information such asa motion vector. The second bitstream generator 113 performs the entropycoding using the variable length coding and the arithmetic coding forthe derived value of each syntax element, according to the syntax rule,generates a second bitstream, and supplies the coded second bitstream tothe bitstream multiplexer 115. The detailed process content for thederivation of the syntax element regarding the intra-brightnessprediction mode and the intra-color-difference prediction mode performedby the second bitstream generator 113 will be described below.

The third bitstream generator 114 performs the entropy coding using thevariable length coding and the arithmetic coding for the orthogonallytransformed and quantized residual signal, according to the prescribedsyntax rule, generates a third bitstream, and supplies the thirdbitstream to the bitstream multiplexer 115.

The first bitstream, the second bitstream, and the third bitstream aremultiplexed by the bitstream multiplexer 115 according to the prescribedsyntax rule, a bit stream is generated, and the multiplexed bit streamis output.

The inverse quantizer/inverse orthogonal transformer 108 performsinverse quantization and inverse orthogonal transform for theorthogonally transformed and quantized residual signal supplied from theorthogonal transformer/quantizer 107, derives a residual signal, andsupplies the residual signal to the decoding picture signal superimposer109. The decoding picture signal superimposer 109 superimposes theprediction signal of the intra-prediction or the inter-predictionaccording to the determination by the coding method determiner 105 andthe residual signal inversely quantized and inversely orthogonallytransformed by the inverse quantizer/inverse orthogonal transformer 108,generates a decoding picture, and stores the decoding picture in thedecoding picture memory 111. A filtering process to decrease blockdistortion by coding may be executed on the decoding picture and thedecoding picture may be stored in the decoding picture memory 111.

FIG. 2 is a block diagram illustrating a configuration of a picturedecoding device according to the embodiment corresponding to the picturecoding device of FIG. 1. The picture decoding device according to theembodiment includes a bitstream separator 201, a first bitstream decoder202, a second bitstream decoder 203, a third bitstream decoder 204, acolor-difference format manager 205, an intra-predictor 206, aninter-predictor 207, an inverse quantizer/inverse orthogonal transformer208, a decoding picture signal superimposer 209, a coding informationmemory 210, a decoding picture memory 211, and switches 212 and 213.

A bit stream supplied to the bitstream separator 201 is separatedaccording to the prescribed syntax rule, a first bitstream showingcoding information of sequence, picture, and slice units is supplied tothe first bitstream decoder 202, a second bitstream including codinginformation of a coding block unit is supplied to the second bitstreamdecoder 203, and a third bitstream including the orthogonallytransformed and quantized residual signal is supplied to the thirdbitstream decoder 204.

The first bitstream decoder 202 performs entropy decoding on thesupplied first bitstream according to the syntax rule and obtains eachvalue of syntax elements regarding coding information of sequence,picture, and slice units. The first bitstream decoder 202 derives thecoding information of the sequence, picture, and slice units from thedecoded value of the syntax element regarding the coding information ofthe sequence, picture, and slice units, according to a semantics ruledefining a meaning of the syntax element and a derivation method. Thefirst bitstream decoder 202 is a bitstream decoder corresponding to thefirst bitstream generator 112 of the coding side and has a function ofreturning the first bitstream including the coding information of thesequence, picture, and slice units coded by the first bitstreamgenerator 112 to each coding information. The color-difference formatinformation coded by the first bitstream generator 112 is derived fromthe value of the syntax element regarding the color-difference formatinformation obtained by performing the entropy decoding on the secondbitstream by the first bitstream decoder 202. The type of thecolor-difference format is specified from the value of the syntaxelement chroma_format_idc according to the syntax rule and the semanticsrule illustrated in FIG. 10 and the syntax element chroma_format_idcshows a monochrome when a value is 0, 4:2:0 when the value is 1, 4:2:2when the value is 2, and 4:4:4 when the value is 3. In addition, whenthe value of the syntax element chroma_format_idc is 3, that is, thecolor-difference format is 4:4:4, the syntax elementseparate_colour_plane_flag is decoded and it is determined whether thebrightness signal and the color-difference signal are coded separately.The derived color-difference format information is supplied to thecolor-difference format manager 205.

The color-difference format manager 205 manages the suppliedcolor-difference format information. The supplied color-differenceformat information is supplied to the second bitstream decoder 203 and aderivation process of the coding information of the coding block and theprediction block based on the color-difference format information isexecuted. Although not illustrated clearly in the drawings, the decodingprocess based on the color-difference format information is executed inthe third bitstream decoder 204 and the intra-predictor 206, theinter-predictor 207, the inverse quantizer/inverse orthogonaltransformer 208, and the decoding picture signal superimposer 209 ofFIG. 2 and management based on the color-difference format informationis performed in the coding information memory 210 and the decodingpicture memory 211.

The second bitstream decoder 203 performs the entropy decoding on thesupplied first bitstream according to the syntax rule and obtains eachvalue of syntax elements regarding coding information of coding blockand prediction block units. The second bitstream decoder 203 derives thecoding information of the coding block unit and the prediction blockunit from the supplied value of the syntax element regarding the codinginformation of the coding block unit and the prediction block unit,according to the semantics rule defining the meaning of the syntaxelement and the derivation method. The second bitstream decoder 203 is abitstream decoder corresponding to the second bitstream generator 113 ofthe coding side and has a function of returning the second bitstreamincluding the coding information of the coding block unit and theprediction block unit coded by the second bitstream generator 113 toeach coding information. Specifically, in addition to the divisionmethod of the coding block, the prediction mode (PredMode), and thedivision mode (PartMode), an intra-prediction mode including anintra-brightness prediction mode and an intra-color-differenceprediction mode is obtained from each syntax element obtained bydecoding the second bitstream according to the prescribed syntax rule,when the prediction mode (PredMode) is the intra-prediction. Meanwhile,when the prediction mode (PredMode) is the inter-prediction, aninter-prediction mode, information specifying a reference picture, andinter-prediction information such as a motion vector are obtained. Whenthe prediction mode (PredMode) is the intra-prediction, theintra-prediction mode including the intra-brightness prediction mode andthe intra-color-difference prediction mode is supplied to theintra-predictor 206 through the switch 212 and when the prediction mode(PredMode) is the inter-prediction, the inter-prediction mode, theinformation specifying the reference picture, and the inter-predictioninformation such as the motion vector are supplied to theinter-predictor 207 through the switch 212. The detailed process for theentropy decoding process and the derivation process of theintra-brightness prediction mode and the intra-color-differenceprediction mode from the syntax element regarding the intra-brightnessprediction mode and the intra-color-difference prediction mode, executedby the second bitstream decoder 203, will be described below.

The third bitstream decoder 204 decodes the supplied third bitstream,derives an orthogonally transformed/quantized residual signal, andsupplies the orthogonally transformed/quantized residual signal to theinverse quantizer/inverse orthogonal transformer 208.

The intra-predictor 206 generates a prediction picture signal by theintra-prediction from the surrounding decoded block stored in thedecoding picture memory 211, according to the supplied intra-brightnessprediction mode and intra-color-difference prediction mode, and suppliesthe prediction picture signal to the decoding picture signalsuperimposer 209 through the switch 213. However, in this embodiment,when the intra-prediction in which the color-difference format is 4:2:2is performed, a second intra-color-difference prediction mode to bedescribed below is used. The intra-prediction and the intra-predictionmode of the color-difference signal will be described in detail below.

The inter-predictor 207 generates a prediction picture signal by theinter-prediction using the motion compensation, from the decodedreference picture stored in the decoding picture memory 211, using theinter-prediction information such as the inter-prediction mode, theinformation specifying the reference picture, and the motion vector tobe supplied, and supplies the prediction picture signal to the decodingpicture signal superimposer 209 through the switch 213. When bothpredictions are performed, the two motion compensation predictionpicture signals of the L0 prediction and the L1 prediction areadaptively multiplied with a weighting coefficient and are superimposedand a final prediction picture signal is generated.

The inverse quantizer/inverse orthogonal transformer 208 performsinverse orthogonal transform and inverse quantization on theorthogonally transformed/quantized residual signal decoded by the thirdbitstream decoder 204 and obtains an inversely orthogonallytransformed/inversely quantized residual signal.

The decoding picture signal superimposer 209 superimposes the predictionpicture signal predicted by the intra-predictor 206 or theinter-predictor 207 and the residual signal inversely orthogonallytransformed/inversely quantized by the inverse quantizer/inverseorthogonal transformer 208, decodes the decoding picture signal, andstores the decoding picture signal in the decoding picture memory 211.When the decoding picture signal is stored in the decoding picturememory 211, a filtering process to decrease block distortion by codingmay be executed on the decoding picture and the decoding picture may bestored in the decoding picture memory 211. The decoding picture signalstored in the decoding picture memory 211 is output in output order.

Next, the intra prediction performed by the intra-predictor 103 of thepicture coding device of FIG. 1 and the intra-prediction performed bythe intra-predictor 206 of the picture decoding device of FIG. 2 and theintra-prediction mode used at the time of the intra-prediction, coded bythe second bitstream generator 113 of FIG. 1, and decoded by the secondbitstream decoder 203 of FIG. 2 will be described.

In the intra-prediction, a value of a pixel of a conversion block of aprocess target is predicted from a value of a pixel of a surroundingdecoded conversion block in the same screen. In the coding device andthe decoding device according to this embodiment, an intra-predictionmode is selected from 35 intra-prediction modes and the intra-predictionis performed. FIG. 8 is a diagram illustrating a value and a predictiondirection of an intra-prediction mode defined in this embodiment. Anarrow direction shows a prediction direction of each intra-prediction,that is, a direction referred to by the intra-prediction. In eachintra-prediction mode, the intra-prediction of each pixel (pixel of astarting point of an arrow of FIG. 8) is performed by referring to adecoded boundary pixel of a prediction direction (arrow direction ofFIG. 8) of the intra-prediction included in a conversion blockneighboring a conversion block becoming a target of theintra-prediction. Each of left and upper numbers shows a value of anintra-prediction mode. Each of right and lower numerals shows an angleof the intra-prediction corresponding to each of the left and upperintra-prediction modes. An intra-prediction mode (intraPredMode) defines33 angle predictions (intra-prediction mode intraPredMode=2 . . . 34) inwhich prediction is performed from a pixel of a surrounding decodedconversion block at various angles, in addition to plane prediction(intra-prediction mode intraPredMode=0) in which prediction is performedby interpolating a pixel value from a pixel of a surrounding decodedconversion block and average value prediction (intra-prediction modeintraPredMode=1) in which prediction is performed by deriving an averagevalue from a pixel of a surrounding decoded conversion block. In theangle predictions, vertical prediction (intra-prediction modeintraPredMode=26) in which prediction is performed from a pixel of anupper decoded conversion block in a vertical direction and horizontalprediction (intra-prediction mode intraPredMode=10) in which predictionis performed from a pixel of a left decoded conversion block in ahorizontal direction are included. In this embodiment, an angle of theintra-prediction is represented by a length of the vertical direction toa unit length 32 of the horizontal direction or a length of thehorizontal direction to a unit length 32 of the vertical direction. Anangle of the intra-prediction corresponding to an intra-prediction modeof the horizontal prediction to perform the prediction in the horizontaldirection is set as 0 and the length of the vertical direction to theunit length 32 of the horizontal direction is represented by a positivevalue in a downward direction and a negative value in an upwarddirection and is set as an angle of the intra-prediction. In addition,an angle of the intra-prediction corresponding to an intra-predictionmode of the vertical prediction to perform the prediction in thevertical direction is set as 0 and the length of the horizontaldirection to the unit length 32 of the vertical direction is representedby a positive value in a rightward direction and a negative value in aleftward direction and is set as an angle of the intra-prediction. Forexample, an angle of the intra-prediction of 32 shows 45° of a degreemeasure and an angle of the intra-prediction of −32 shows −45° of thedegree measure.

The intra-prediction mode is prepared for each of the brightness signaland the color-difference signal and an intra-prediction mode for thebrightness signal is defined as an intra-brightness prediction mode andan intra-prediction mode for the color-difference signal is defined asan intra-color-difference prediction mode. When the intra-brightnessprediction mode is coded and decoded, a correlation with anintra-brightness prediction mode of a neighboring block is used. When itis determined that prediction can be performed from the intra-brightnessprediction mode of the neighboring block at the coding side, informationspecifying a reference block is transmitted. When it is determined thatsetting a different value to the intra-brightness prediction mode ismore preferable than the prediction from the intra-brightness predictionmode of the neighboring block, a mechanism for coding or decoding thevalue of the intra-brightness prediction mode is used. Anintra-brightness prediction mode of a coding/decoding target block ispredicted from the intra-brightness prediction mode of the neighboringblock, so that a transmitted code amount can be reduced. Meanwhile, whenthe intra-color-difference prediction mode is coded and decoded, acorrelation with an intra-brightness prediction mode of a predictionblock of a brightness signal of the same position as the predictionblock of the color-difference signal is used. When it is determined thatprediction can be performed from the intra-brightness prediction mode atthe coding side, a value of the intra-color-difference prediction modeis predicted from a value of the intra-brightness prediction mode. Whenit is determined that setting an independent value to theintra-color-difference prediction mode is more preferable than theprediction from the intra-brightness prediction mode, a mechanism forcoding or decoding the value of the intra-color-difference predictionmode is used. An intra-color-difference prediction mode is predictedfrom the intra-brightness prediction mode, so that a transmitted codeamount can be reduced.

Next, a coding process of coding information of the coding block andprediction block units executed by the second bitstream generator 113 ofFIG. 1 will be described on the basis of a point relating to theintra-prediction mode to be a characteristic of the embodiment. FIG. 12is a block diagram illustrating a configuration of the second bitstreamgenerator 113 of FIG. 1.

As illustrated in FIG. 12, the second bitstream generator 113 of FIG. 1includes a syntax element deriver 121 regarding coding information of acoding block unit, a syntax element deriver 122 regarding anintra-brightness prediction mode, a syntax element deriver 123 regardingan intra-color-difference prediction mode, a syntax element deriver 124regarding inter-prediction information, an intra-prediction mode codingcontroller 125, and an entropy coder 126. In individual elementsconfiguring the second bitstream generator 113, a process according tocolor difference information supplied from the color-difference formatsetter 101 is executed and a process according to coding informationsuch as a prediction mode and a division mode (PartMode) of a codingblock unit is executed.

The syntax element deriver 121 regarding the coding information of thecoding block unit derives a value of a syntax element regarding thecoding information of the coding block unit and supplies the derivedvalue of each syntax element to the entropy coder 126. Values of syntaxelements regarding a prediction mode (PredMode) to determineintra-prediction (MODE_INTRA) or inter-prediction (MODE_INTER) of thecoding block and a division mode (PartMode) to determine a shape of aprediction block are derived by the syntax element deriver 121 regardingthe coding information of the coding block unit.

When the prediction mode (PredMode) of the coding block is theintra-prediction (MODE_INTRA), the syntax element deriver 122 regardingthe intra-brightness prediction mode derives a value of a syntax elementregarding an intra-brightness prediction mode of a prediction block of abrightness signal and supplies the derived value of each syntax elementto the entropy coder 126. The syntax elements regarding theintra-brightness prediction mode are a syntax elementprev_intra_luma_pred_flag[x0][y0] to be a flag showing whetherprediction can be performed from an intra-brightness prediction mode ofa neighboring block, a syntax element mpm_idx[x0][y0] to be an indexshowing a prediction block of a prediction origin, and a syntax elementrem_intra_luma_pred_mode[x0][y0] showing an intra-brightness predictionmode of a prediction block unit. In addition, x0 and y0 are thecoordinates showing a position of the prediction block. When the valueof the syntax element regarding the intra-brightness prediction mode isderived, a correlation with an intra-brightness prediction mode of aneighboring block stored in the coding information memory 110 is used.When prediction can be performed from the intra-brightness predictionmode of the neighboring block, a syntax elementprev_intra_luma_pred_flag [x0][y0] to be a flag showing that the valueis used is set to 1 (true) and a value to specify a referencedestination is set to the syntax element mpm_idx[x0][y0] to be thesyntax showing the prediction block of the prediction origin. When theprediction cannot be performed, prev_intra_luma_pred_flag [x0][y0] isset to 0 (false) and a value to specify the intra-brightness predictionmode is set to the syntax element rem_intra_luma_pred_mode[x0][y0]showing an intra-brightness prediction mode to be coded.

The number of intra-brightness prediction modes of a prediction block ina coding block is different according to a division block. When thedivision mode (PartMode) is 2N×2N division, values of syntax elementsregarding intra-brightness prediction modes of one set of predictionblocks are derived for each coding block and when the division mode isN×N division, values of syntax elements regarding intra-brightnessprediction modes of four sets of prediction blocks are derived for eachcoding block.

When the prediction mode (PredMode) of the coding block is theintra-prediction (MODE_INTRA), the syntax element deriver 123 regardingthe intra-color-difference prediction mode derives a value of a syntaxelement intra_chroma_pred_mode[x0][y0] regarding anintra-color-difference prediction mode of a prediction block of acolor-difference signal and supplies the derived value of the syntaxelement intra_chroma_pred_mode[x0][y0] to the entropy coder 126. Indetermination of the intra-color-difference prediction mode in theintra-predictor 103 and derivation of the value of the syntax elementregarding the intra-color-difference prediction mode of the syntaxelement deriver 123 regarding the intra-color-difference predictionmode, a correlation with an intra-brightness prediction mode of aprediction block of a brightness signal of the same position as theprediction block of the color-difference signal is used. When aprediction value from the intra-brightness prediction mode of theprediction block of the brightness signal of the same position as theprediction block of the color-difference signal is most suitable, avalue of the intra-color-difference prediction mode is predicted fromthe value of the intra-brightness prediction mode. When it is determinedthat setting an independent value is more preferable than the predictionvalue from the intra-brightness prediction mode, a mechanism for settingany value of 0 (plane prediction), 1 (average value prediction), 10(horizontal prediction), 26 (vertical prediction), and 34 (obliqueprediction) to be representative intra-prediction modes to theintra-color-difference prediction mode is used and a code amount isreduced.

Here, a method of deriving a value of the intra-color-differenceprediction mode from the value of the intra-brightness prediction modeand the value of the syntax element regarding the intra-color-differenceprediction mode by the intra-color-difference prediction mode deriver225 to be described below at the decoding side will be described. Inthis embodiment, an intra-color-difference prediction mode for 4:2:0 or4:4:4 derived by a table of FIG. 14 to be described below is defined asa first intra-color-difference prediction mode to distinguish anintra-color-difference prediction mode for a color-difference format4:2:2 to be described below and the intra-color-difference predictionmode. FIG. 14 is a table to derive a value of a firstintra-color-difference prediction mode from a value of a syntax elementintra_chroma_pred_mode[x0][y0] regarding an intra-color-differenceprediction mode defined in this embodiment and a value of anintra-brightness prediction mode of a prediction block of the sameposition as a prediction block of a color-difference signal. At thedecoding side, the value of the first intra-color-difference predictionmode is derived using the table.

In the case in which the value of the syntax elementintra_chroma_pred_mode[x0][y0] is 0, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not0, the value of the first intra-color-difference prediction mode takes avalue of 0 (plane prediction) and if the value of the intra-brightnessprediction mode is 0, the value of the first intra-color-differenceprediction mode takes a value of 34 (oblique prediction).

In the case in which the value of the syntax elementintra_chroma_pred_mode[x0][y0] is 1, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not1, the value of the first intra-color-difference prediction mode takes avalue of 26 (vertical prediction) and if the value of theintra-brightness prediction mode is 1, the value of the firstintra-color-difference prediction mode takes a value of 34 (obliqueprediction).

In the case in which the value of the syntax elementintra_chroma_pred_mode[x0][y0] is 2, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not2, the value of the first intra-color-difference prediction mode takes avalue of 10 (horizontal prediction) and if the value of theintra-brightness prediction mode is 2, the value of the firstintra-color-difference prediction mode takes a value of 34 (obliqueprediction).

In the case in which the value of the syntax elementintra_chroma_pred_mode[x0][y0] is 3, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not3, the value of the first intra-color-difference prediction mode takes avalue of 1 (average value prediction) and if the value of theintra-brightness prediction mode is 3, the value of the firstintra-color-difference prediction mode takes a value of 34 (obliqueprediction).

In the case in which the value of the syntax elementintra_chroma_pred_mode[x0][y0] is 4, the value of the firstintra-color-difference prediction mode takes the same value as theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal.

When the color-difference format is 4:2:0 or 4:4:4, the firstintra-color-difference prediction mode derived by FIG. 14 is used as anintra-prediction mode of a color-difference signal for acolor-difference format 4:2:0 or 4:4:4. In the intra-predictor 103 ofthe picture coding device of FIG. 1 and the intra-predictor 206 of thepicture decoding device of FIG. 2, when the color-difference format is4:2:0 or 4:4:4, the intra-prediction of the color-difference signal isperformed using the first intra-color-difference prediction mode.

When the color-difference format is 4:2:2, a value of anintra-color-difference prediction mode for a color-difference format4:2:2 is derived from the first intra-color-difference prediction modederived by FIG. 14 by the conversion table. In coding and decodingaccording to this embodiment, the intra-color-difference prediction modefor the color-difference format 4:2:2 derived by a conversion table ofFIG. 15, 16, 17, 30, or 33 to be described below is defined as a secondintra-color-difference prediction mode. In the intra-predictor 103 ofthe picture coding device of FIG. 1 and the intra-predictor 206 of thepicture decoding device of FIG. 2, when the color-difference format is4:2:2, the intra-prediction of the color-difference signal is performedusing the second intra-color-difference prediction mode. FIGS. 15, 16,17, 30, and 33 are conversion tables to derive a value of the secondintra-color-difference prediction mode for the color-difference format4:2:2 used for the intra-prediction of the color-difference signal inwhich the color-difference format is 4:2:2, from the intra-brightnessprediction mode defined in this embodiment or the firstintra-color-difference prediction mode derived by the table of FIG. 14.FIG. 18 is a diagram illustrating a value and a prediction direction ofan intra-prediction mode derived by the conversion table of FIG. 15defined in this embodiment. FIG. 19 is a diagram illustrating a valueand a prediction direction of an intra-prediction mode derived by theconversion table of FIG. 16 defined in this embodiment. FIG. 20 is adiagram illustrating a value and a prediction direction of anintra-prediction mode derived by the conversion table of FIG. 17 definedin this embodiment. FIG. 31 is a diagram illustrating a value and aprediction direction of an intra-prediction mode derived by theconversion table of FIG. 30 defined in this embodiment. FIG. 34 is adiagram illustrating a value and a prediction direction of anintra-prediction mode derived by the conversion table of FIG. 33 definedin this embodiment.

In this embodiment, a process for deriving the secondintra-color-difference prediction mode for the color-difference format4:2:2 from the intra-brightness prediction mode or the firstintra-color-difference prediction mode in FIGS. 15, 16, 17, 30, and 33is executed by the intra-predictor 103 of the coding device at thecoding side and is executed by the second bitstream decoder 203 or theintra-predictor 206 of the decoding device at the decoding side.

The reason why the second intra-color-difference prediction mode for thecolor-difference format 4:2:2 is derived using the conversion table ofFIG. 15, 16, 17, 30, or 33, instead of using directly the firstintra-color-difference prediction mode derived by the table of FIG. 14like 4:2:0 or 4:4:4, when the color-difference format is 4:2:2, incoding and decoding according to this embodiment, will be described.When the color-difference format is 4:2:2, the color-difference formatis a color-difference format in which the color-difference signal issampled at a density of ½ in a horizontal direction and the same densityin a vertical direction, with respect to the brightness signal, asillustrated in FIG. 3B. Therefore, if the intra-prediction of thecolor-difference signal is performed in a prediction direction obtainedby scaling a prediction direction of each of the intra-brightnessprediction mode and the first intra-color-difference prediction mode ½times in the horizontal direction or a surrounding prediction directionthereof, the intra-prediction becomes equivalent to or almost equivalentto the intra-prediction of the brightness signal of the prediction blockof the same position as the prediction block of the color-differencesignal.

This will be described in detail with reference to FIGS. 21A and 21B.FIGS. 21A and 21B are a diagram illustrating a correspondence relationof prediction directions of intra-predictions of a brightness signal anda color-difference signal when a color-difference format is 4:2:2. InFIGS. 21A and 21B, x shows a position of a pixel of a brightness signaland ∘ shows a position of a pixel of a color-difference signal. In4:2:2, a color-difference signal is sampled at a density of ½ in ahorizontal direction, with respect to a brightness signal, and aspectratios of pixels of the brightness signal and the color-differencesignal are different from each other. FIG. 21A illustrates positions ofsampled pixels of a brightness signal and a color-difference signal at4:2:2. A symbol P1 shows a pixel of the intra prediction and a symbol P2shows a pixel (in actuality, because filtering is performed, adjacentpixels are also referred to) to be referred to at the time of theintra-prediction. An arrow extended from a pixel P1 to a pixel P2, shownby a symbol 2701, shows an intra-prediction direction of the pixel P1 ofthe brightness signal and shows an intra-prediction direction of thepixel P1 of the color-difference signal.

FIG. 21B illustrates an array of pixels of a color-difference signalsampled at a density of ½ in the horizontal direction. Here, whenscaling of ½ is not performed in the horizontal direction at the time ofthe intra-prediction of the color-difference signal, theintra-prediction direction of the pixel P1 of the color-differencesignal becomes an arrow direction shown by a symbol 2702 and a pixel ofa symbol P3 is erroneously referred to in the pixel array of thecolor-difference signal. However, a correct reference destination is thepixel shown by the symbol P2. Therefore, the intra-prediction directionof the brightness signal is scaled ½ times in the horizontal directionand the intra-prediction direction is set to the intra-predictiondirection of the color-difference signal, so that a correctintra-prediction direction in the array of the color-difference signalis derived, as shown by a symbol 2703, and a pixel (in actuality,because filtering is performed, adjacent pixels are also referred to) tobe the correct reference destination in the intra-prediction directionand neighboring in an upward direction is acquired.

In FIGS. 21A and 21B, the case in which the pixel neighboring theprediction block in the upward direction is referred to has beendescribed. However, the case in which a pixel neighboring in a leftwarddirection is referred to is also the same. In the case of the pixelneighboring in the leftward direction, the intra-prediction direction ofthe brightness signal is scaled two times in the vertical direction(this is equivalent to scaling the intra-prediction direction ½ times inthe horizontal direction, from the viewpoint of acquiring the directionof the intra-prediction), so that a correction intra-predictiondirection in the array of the color-difference signal is derived, and apixel (a part of pixels neighboring in the upward direction is alsoincluded) to be the correct reference destination in theintra-prediction direction and neighboring in the leftward direction isacquired.

Therefore, in the conversion tables of FIGS. 15 and 16, as shown byarrows of dotted lines of FIGS. 18 and 19, when values of theintra-brightness prediction modes arranged in the horizontal direction(on a horizontal axis) or the first intra-color-difference predictionmodes derived by the table of FIG. 14 are 18, 19, 20, 21, 22, 23, 24,25, 26, 27, 28, 29, 30, 31, 32, 33, and 34, values of intra-predictionmodes of a prediction direction close to a prediction direction derivedby scaling an angle of the prediction direction ½ times in thehorizontal direction on the basis of the vertical prediction(intra-prediction mode 26) are selected as values of the secondintra-color-difference prediction modes and the values of the secondintra-color-difference prediction modes are set to 21, 22, 23, 23, 24,24, 25, 25, 26, 27, 27, 28, 28, 29, 29, 30, and 31, respectively. Inaddition, scaling the prediction direction of the intra-prediction ½times in the horizontal direction is equivalent to scaling theprediction direction two times in the vertical direction. Therefore, ifthe intra-prediction of the color-difference signal is performed in aprediction direction obtained by scaling a prediction direction of eachof the intra-brightness prediction mode and the firstintra-color-difference prediction mode two times in the verticaldirection on the basis of the horizontal prediction (intra-predictionmode 10) or a surrounding prediction direction thereof, theintra-prediction becomes equivalent to or almost equivalent to theintra-prediction of the brightness signal of the prediction block of thesame position as the prediction block of the color-difference signal.Therefore, in the conversion tables of FIGS. 15 and 16, as illustratedin FIGS. 18 and 19, when values of the intra prediction modes(intra-brightness prediction modes or first intra-color-differenceprediction modes) arranged in the vertical direction (on a verticalaxis) are 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, and 17,values of intra-prediction modes of a prediction direction close to aprediction direction derived by scaling an angle of the predictiondirection two times in the vertical direction on the basis of thehorizontal prediction (intra-prediction mode 10) are selected as valuesof the second intra-color-difference prediction modes and the values ofthe second intra-color-difference prediction modes are set to 2, 2, 2,2, 3, 5, 7, 8, 10, 12, 13, 15, 17, 18, 18, 18, and 18 and 2, 2, 2, 2, 3,5, 7, 8, 10, 12, 13, 15, 17, 18, 18, 19, and 20, respectively.

In addition, the intra-prediction mode (intra-brightness prediction modeor first intra-color-difference prediction mode) can be converted intothe second intra-color-difference prediction mode using the conversiontable of FIG. 17. In the conversion table of FIG. 17, as shown by anarrow of a dotted line of FIG. 20, when values of the intra-brightnessprediction modes in which the reference destinations are arranged in thehorizontal direction (on a horizontal axis) or the firstintra-color-difference prediction modes derived by the table of FIG. 14are 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, and34, values derived by scaling the intra-brightness prediction mode orthe first intra-color-difference prediction mode ½ times in thehorizontal direction on the basis of the vertical prediction(intra-prediction mode 26) are set to values of the secondintra-color-difference prediction modes and the values of the secondintra-color-difference prediction modes are set to 22, 22, 23, 23, 24,24, 25, 25, 26, 27, 27, 28, 28, 29, 29, 30, and 30, respectively. Inaddition, scaling the prediction direction of the intra-prediction ½times in the horizontal direction is equivalent to scaling theprediction direction two times in the vertical direction. Therefore, ifthe intra-prediction of the color-difference signal is performed in aprediction direction obtained by scaling a prediction direction of eachof the intra-brightness prediction mode and the firstintra-color-difference prediction mode two times in the verticaldirection on the basis of the horizontal prediction (intra-predictionmode 10) or a surrounding prediction direction thereof, theintra-prediction becomes equivalent to or almost equivalent to theintra-prediction of the brightness signal of the prediction block of thesame position as the prediction block of the color-difference signal.Therefore, in the conversion table of FIG. 17, as shown by an arrow of adotted line of FIG. 20, when values of the intra-brightness predictionmodes in which the reference destinations are arranged in the verticaldirection (on a vertical axis) or the first intra-color-differenceprediction modes are 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,and 17, values derived by scaling the intra-brightness prediction modeor the first intra-color-difference prediction mode two times in thevertical direction on the basis of the horizontal prediction(intra-prediction mode 10) and limiting the values to larger than orequal to 2 and smaller than or equal to 18 are set to values of theintra-color-difference prediction modes and the values of the secondintra-color-difference prediction modes are set to 2, 2, 2, 2, 2, 4, 6,8, 10, 12, 14, 16, 18, 18, 18, and 18.

In addition, the intra-prediction mode (intra-brightness prediction modeor first intra-color-difference prediction mode) can be converted intothe second intra-color-difference prediction mode using the conversiontable of FIG. 30. In the conversion table of FIG. 30, as shown by anarrow of a dotted line of FIG. 31, when values of the intra-brightnessprediction modes or the first intra-color-difference prediction modesderived by the table of FIG. 14 are 16, 17, 18, 19, 20, 21, 22, 23, 24,25, 26, 27, 28, 29, 30, 31, 32, 33, and 34, values derived by scalingthe intra-brightness prediction mode or the first intra-color-differenceprediction mode ½ times in the horizontal direction on the basis of thevertical prediction (intra-prediction mode 26) are set to values of thesecond intra-color-difference prediction modes and the values of thesecond intra-color-difference prediction modes are set to 21, 21, 22,22, 23, 23, 24, 24, 25, 25, 26, 27, 27, 28, 28, 29, 29, 30, and 30,respectively. In addition, when values of the intra-brightnessprediction modes or the first intra-color-difference prediction modesare 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15, the valuesderived by scaling the intra-brightness prediction modes or the firstintra-color-difference prediction modes two times in the verticaldirection on the basis of the horizontal prediction (intra-predictionmode 10) and limiting the values to larger than or equal to 2 are set tovalues of the second intra-color-difference prediction modes and thevalues of the second intra-color-difference prediction modes are set to2, 2, 2, 2, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20.

In addition, the intra-prediction mode (intra-brightness prediction modeor first intra-color-difference prediction mode) can be converted intothe second intra-color-difference prediction mode using the conversiontable of FIG. 33. In the conversion table of FIG. 33, as shown by anarrow of a dotted line of FIG. 34, when values of the intra-brightnessprediction modes or the first intra-color-difference prediction modesderived by the table of FIG. 14 are 21, 22, 23, 24, 25, 26, 27, 28, 29,30, and 31, values derived by scaling the intra-brightness predictionmode or the first intra-color-difference prediction mode ½ times in thehorizontal direction on the basis of the vertical prediction(intra-prediction mode 26) are set to values of the secondintra-color-difference prediction modes and the values of the secondintra-color-difference prediction modes are set to 23, 24, 24, 25, 25,26, 27, 27, 28, 28, and 29, respectively. In addition, when values ofthe intra-brightness prediction modes or the firstintra-color-difference prediction modes are 7, 8, 9, 10, 11, 12, and 13,the values derived by scaling the intra-brightness prediction modes orthe first intra-color-difference prediction modes two times in thevertical direction on the basis of the horizontal prediction(intra-prediction mode 10) are set to values of the secondintra-color-difference prediction modes and the values of the secondintra-color-difference prediction modes are set to 4, 6, 8, 10, 12, 14,and 16. In addition, when values of the intra-brightness predictionmodes or the first intra-color-difference prediction modes are 2, 3, 4,5, and 6, the values derived by subtracting 3 from the intra-brightnessprediction modes or the first intra-color-difference prediction modesand limiting the values to larger than or equal to 2 are set to valuesof the second intra-color-difference prediction modes and the values ofthe second intra-color-difference prediction modes are set to 2, 2, 2,2, and 3. In addition, when values of the intra-brightness predictionmodes or the first intra-color-difference prediction modes are 14, 15,16, 17, 18, 19, and 20, the values derived by adding 3 to theintra-brightness prediction modes or the first intra-color-differenceprediction modes are set to values of the second intra-color-differenceprediction modes and the values of the second intra-color-differenceprediction modes are set to 17, 18, 19, 20, 21, and 22. In addition,when values of the intra-brightness prediction modes or the firstintra-color-difference prediction modes are 32, 33, and 34, the valuesderived by subtracting 3 from the intra-brightness prediction modes orthe first intra-color-difference prediction modes are set to values ofthe second intra-color-difference prediction modes and the values of thesecond intra-color-difference prediction modes are set to 29, 30, and31.

A derivation process sequence when the first intra-color-differenceprediction mode corresponding to the conversion tables of FIGS. 15 and16 when the first intra-color-difference prediction mode is convertedinto the second intra-color-difference prediction mode at thecolor-difference format 4:2:2 is converted into the secondintra-color-difference prediction mode will be described using aflowchart of FIG. 23.

In each value of a first intra-prediction mode IntraPredMode1 from 0 to34, a second intra-prediction mode IntraPredMode2 is derived by thesequence of the flowchart of FIG. 23.

First, when the prediction is not the angle prediction, that is, thefirst intra-prediction mode IntraPredMode1 is smaller than or equal to 1(NO of step S3001 of FIG. 23), the value of the first intra-predictionmode IntraPredMode1 is used directly as the secondintra-color-difference prediction mode IntraPredMode2 (step S3002 ofFIG. 23) and this derivation process ends. When the firstintra-prediction mode IntraPredMode1 is smaller than or equal to 1, thiscorresponds to plane prediction (intra-prediction mode intraPredMode1=0)in which prediction is performed by interpolating a pixel value from asurrounding decoded block and average value prediction (intra-predictionmode intraPredMode1=1) in which prediction is performed by deriving anaverage value from a surrounding decoded block.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is the angle prediction, that is, larger than 1 (YES ofstep S3001 of FIG. 23), a conversion process from the firstintra-color-difference prediction mode to the secondintra-color-difference prediction mode after step S3003 is executed.

When the intra-color-difference prediction mode IntraPredMode1 issmaller than 18 (YES of step S3003 of FIG. 23), an angle IntraPredAngle1of the first intra-prediction corresponding to the firstintra-color-difference prediction mode IntraPredMode1 is doubled and isset to an angle IntraPredAngle2′ of the intra-prediction (step S3004 ofFIG. 23). In addition, a value of the intra-brightness prediction modeor and the first intra-color-difference prediction mode IntraPredMode1corresponding to the angle IntraPredAngle1 of the first intra-predictionclose to the angle IntraPredAngle2′ of the intra-prediction is set tothe second intra-prediction mode IntraPredMode2 (step S3005 of FIG. 23)and this derivation process ends. However, when the firstintra-prediction modes are 2, 3, 4, and 5, a value of the angleIntraPredAngle2′ of the intra-prediction becomes smaller than or equalto −32. However, at this time, the angle of the intra-prediction is setto −32 and the second intra-color-difference prediction mode is set to2. When the first intra-prediction modes are 15, 16, and 17, a value ofthe angle intraPredAngle2′ of the Intra-prediction becomes larger thanor equal to 32. However, at this time, the angle of the intra-predictionis set to 32 and the second intra-color-difference prediction mode isset to 18. As illustrated in FIG. 19, if the angle of theintra-prediction corresponding to the first intra-prediction mode of 16is doubled in a longitudinal direction, the angle takes a value close toan angle of the intra-prediction corresponding to the intra-predictionmode of 19 and if the angle of the intra-prediction corresponding to thefirst intra-prediction mode of 17 is doubled in a longitudinaldirection, the angle takes a value close to an angle of theintra-prediction corresponding to the intra-prediction mode of 20.Therefore, as illustrated in FIG. 19, when the first intra-predictionmode is 16, the second intra-prediction mode can be set to 19 and whenthe first intra-prediction mode is 17, the second intra-prediction modecan be set to 20.

Meanwhile, when the intra-color-difference prediction modeIntraPredMode1 is not smaller than 18, that is, larger than or equal to18 (NO of step S3003 of FIG. 23), the angle IntraPredAngle1 of the firstintra-prediction corresponding to the first intra-color-differenceprediction mode IntraPredMode1 is multiplied with ½ and is set to theangle IntraPredAngle2′ of the intra-prediction (steps S3006 and S3007 ofFIG. 23). In this embodiment, a value is set to a variableSignIntraPredAngle having a value of −1 when a sign of the angleIntraPredAngle1 of the first intra-prediction corresponding to the firstintra-color-difference prediction mode IntraPredMode1 is negative andhaving a value of 1 when the sign is positive or a variable a is 0 (stepS3006 of FIG. 23), a result obtained by executing an operation to shiftone bit to the right, equivalent to ½ times, on an absolute value of thefirst intra-color-difference prediction mode IntraPredMode1 ismultiplied with the variable SignIntraPredAngle, and an obtained valueis set to the angle IntraPredAngle2′ of the intra-prediction (step S3007of FIG. 23). Also, a result obtained by executing the operation to shiftone bit to the right, equivalent to ½ times, on the absolute value ofthe first intra-color-difference prediction mode IntraPredMode1 afteradding 1 to the absolute value may be multiplied with the variableSignIntraPredAngle and an obtained value may be set to the angleIntraPredAngle2′ of the intra-prediction. In addition, a value of anintra-prediction mode (upper step of FIG. 8) corresponding to an angle(lower step of FIG. 8) of the intra-prediction prepared in theintra-brightness prediction mode and the first intra-color-differenceprediction mode, close to the angle IntraPredAngle2′ of theintra-prediction, is set to the second intra-prediction modeIntraPredMode2 (step S3008 of FIG. 23) and this derivation process ends.When the angle IntraPredAngle2′ of the intra-prediction is rounded offto a value of an angle of the intra-prediction prepared in theintra-brightness prediction mode and the first intra-color-differenceprediction mode, the angle IntraPredAngle2′ of the intra-prediction maybe rounded off to a value closest to the angle IntraPredAngle2′ of theintra-prediction, may be revalued, and may be devalued. In addition, thesame positive/negative sign as the angle IntraPredAngle2′ of theintra-prediction may be set to a value obtained by rounding off,revaluing, or devaluing an absolute value of the angle IntraPredAngle2′of the intra-prediction.

If the angle of the intra-prediction in which the firstintra-color-difference prediction mode is 25 is multiplied with ½, theangle becomes −1 and −1 can be converted into a value of either −2corresponding to the value of the intra-prediction mode of 25 or 0corresponding to the value of the intra-prediction mode of 26. However,because coding can be performed at all times in 26 showing the verticalprediction, the first intra-color-difference prediction mode is set to25 when 25 is converted into the second intra-color-differenceprediction mode. If the angle of the intra-prediction in which the firstintra-color-difference prediction mode is 27 is multiplied with ½, theangle becomes 1 and 1 can be converted into a value of either 0corresponding to the value of the intra-prediction mode of 26 or 2corresponding to the value of the intra-prediction mode of 27. However,because coding can be performed at all times in 26 showing the verticalprediction, the first intra-color-difference prediction mode is set to27 when 27 is converted into the second intra-color-differenceprediction mode. In other words, when the first intra-color-differenceprediction mode is converted into the second intra-color-differenceprediction mode using the conversion tables illustrated in FIGS. 15, 16,17, 30, and 33, in the case in which the first intra-color-differenceprediction mode is not 26 to be the vertical prediction, the value isconverted into a value other than 26 to be the vertical prediction andthe second intra-color-difference prediction mode is derived from thefirst intra-color-difference prediction mode. That is, the value isconverted to exclude values (0, 1, 10, 26, and 34) that can be derivedfrom syntax elements (0, 1, 2, and 3 of FIGS. 14 and 25) of theintra-color-difference prediction modes selected when the value of theintra-brightness prediction mode and the value of theintra-color-difference prediction mode are not matched with each other.By this setting, a selection width of the intra-color-differenceprediction mode is extended and coding efficiency can be improved.

In steps S3005 and S3008, when a value of the intra-brightnessprediction mode or the first intra-color-difference prediction modeIntraPredMode1 corresponding to the angle IntraPredAngle1 of the firstintra-prediction close to the angle IntraPredAngle2′ of theintra-prediction is set to the second intra-prediction modeIntraPredMode2 and an intra-prediction operation of the color-differencesignal at the color-difference format 4:2:2 is mounted by hardware, theintra-prediction operation using the second intra-color-differenceprediction mode can be realized by only the angle of theintra-prediction corresponding to the intra-brightness prediction modeor the first intra-color-difference prediction mode IntraPredMode1.Therefore, the intra-prediction operation can be executed without addinghardware by an angle of new intra-prediction.

In the angle predictions, vertical prediction (intra-prediction modeintraPredMode1=26) in which prediction is performed from an upperdecoded block in a vertical direction and horizontal prediction(intra-prediction mode intraPredMode1=10) in which prediction isperformed from a left decoded block in a horizontal direction are alsoincluded. However, in the vertical prediction and the horizontalprediction, even though the conversion process from the firstintra-color-difference prediction mode to the secondintra-color-difference prediction mode after step S3003 is executed,values do not change. Therefore, in condition determination of stepS3001, the process may proceed to step S3002, in the cases of thevertical prediction and the horizontal prediction.

A derivation process sequence when the first intra-color-differenceprediction mode corresponding to the conversion table of FIG. 17 isconverted into the second intra-color-difference prediction mode at thecolor-difference format 4:2:2 will be described using a flowchart ofFIG. 24.

When the prediction is not the angle prediction, that is, the firstintra-prediction mode IntraPredMode1 is smaller than or equal to 1 (NOof step S3101 of FIG. 24), the value of the first intra-prediction modeIntraPredMode1 is used directly as the second intra-color-differenceprediction mode IntraPredMode2 (step S3102 of FIG. 24) and thisderivation process ends.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is the angle prediction, that is, larger than 1 (YES ofstep S3101 of FIG. 24), a conversion process from the firstintra-color-difference prediction mode to the secondintra-color-difference prediction mode after step S3103 is executed.

When the first intra-color-difference prediction mode IntraPredMode1 issmaller than 18 (YES of step S3103 of FIG. 24), a value derived byscaling the first intra-color-difference prediction mode derived by thetable of FIG. 14 two times in the vertical direction on the basis of thehorizontal prediction (intra-prediction mode 10) is set to a value ofthe second intra-color-difference prediction mode (steps S3304 to S3307of FIG. 32). A value obtained by subtracting 10 showing the horizontalprediction from the first intra-color-difference prediction modeIntraPredMode1 is set to a variable a (step S3104 of FIG. 24). Next, avalue obtained by doubling the variable a is set to a variable b (stepS3105 of FIG. 24). Next, a value obtained by adding 10 showing thehorizontal prediction to b is set to a variable c (step S3106 of FIG.24). Next, a value obtained by limiting the value of the variable c tolarger than or equal to 2 and smaller than or equal to 18 is set to thesecond intra-prediction mode IntraPredMode2 (step S3107 of FIG. 24) andthis derivation process ends. Specifically, when the variable c islarger than or equal to 2 and is smaller than 18, the variable c is setdirectly to the value of the intra-prediction mode IntraPredMode2, whenthe variable c is smaller than 2, 2 is set to of the intra-predictionmode IntraPredMode2, and when the variable c is more than 18, 18 is setto the intra-prediction mode IntraPredMode2. That is, when a valuederived by scaling a mode number of the angle prediction of the firstintra-color-difference prediction mode is beyond a range of mode numbersof the angle prediction defined by the intra-prediction mode, thederived value is set to a value in the range. As a result, when anintra-prediction operation of the color-difference signal at thecolor-difference format 4:2:2 is mounted by hardware, theintra-prediction operation using the second intra-color-differenceprediction mode can be executed without adding the hardware.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is not smaller than 18, that is, larger than or equal to18 (NO of step S3103 of FIG. 24), a value derived by scaling the firstintra-color-difference prediction mode derived by the table of FIG. 14 ½times in the horizontal direction on the basis of the verticalprediction (intra-prediction mode 26) is set to a value of the secondintra-color-difference prediction mode (steps S3108 to S3112 of FIG.24). A value obtained by subtracting 26 showing the vertical predictionfrom the first intra-color-difference prediction mode IntraPredMode1 isset to the variable a (step S3108 of FIG. 24). Next, a value is set to avariable SignIntraPredMode having a value of −1 when a sign of thevariable a is negative and a value of 1 when the sign is positive or thevariable a is 0 (step S3109 of FIG. 24). Next, a result obtained byexecuting an operation to shift one bit to the right, equivalent to ½times, on an absolute value of the variable a, is multiplied with thevariable SignIntraPredMode and an obtained value is set to the variableb (step S3110 of FIG. 24). Also, a result obtained by executing theoperation to shift one bit to the right, equivalent to ½ times, on theabsolute value of the variable a after adding 1 to the absolute value,may be multiplied with the variable SignIntraPredMode and an obtainedvalue may be set to the variable b. Next, a value obtained by adding 26showing the vertical prediction to b is set to the variable c (stepS3111 of FIG. 24). Next, the value of the variable c is set to thesecond intra-prediction mode IntraPredMode2 (step S3112 of FIG. 24) andthis derivation process ends. In the case in which the value of thevariable c corresponding to the first intra-color-difference predictionmode of 25 becomes 26, the first intra-color-difference prediction modeis set to 25 when 25 is converted into the second intra-color-differenceprediction mode, excluding 26 showing the vertical prediction in whichcoding can be performed at all times. In the case in which the value ofthe variable c corresponding to the first intra-color-differenceprediction mode of 27 becomes 26, the first intra-color-differenceprediction mode is set to 27 when 27 is converted into the secondintra-color-difference prediction mode, excluding 26 showing thevertical prediction in which coding can be performed at all times. Thatis, the value is rounded off (converted) to exclude values (0, 1, 10,26, and 34) that can be derived from syntax elements (0, 1, 2, and 3 ofFIGS. 14 and 25) of the intra-color-difference prediction modes selectedwhen the value of the intra-brightness prediction mode and the value ofthe intra-color-difference prediction mode are not matched with eachother.

By this setting, a selection width of the intra-color-differenceprediction mode is extended and coding efficiency can be improved. Inthis derivation process sequence, to convert the value to exclude avalue 26 that can be derived from a syntax element (1 of FIGS. 14 and25) of the intra-color-difference prediction mode selected when thevalue of the intra-brightness prediction mode and the value of theintra-color-difference prediction mode are not matched with each other,in step S3110, 1 is added to the absolute value of the variable a,before the operation to shift one bit to the right is executed on theabsolute value of the variable a.

A derivation process sequence when the first intra-color-differenceprediction mode corresponding to the conversion table of FIG. 30 isconverted into the second intra-color-difference prediction mode at thecolor-difference format 4:2:2 will be described using a flowchart ofFIG. 32.

When the prediction is not the angle prediction, that is, the firstintra-prediction mode IntraPredMode1 is smaller than or equal to 1 (NOof step S3301 of FIG. 32), the value of the first intra-prediction modeIntraPredMode1 is used directly as the second intra-color-differenceprediction mode IntraPredMode2 (step S3302 of FIG. 32) and thisderivation process ends.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is the angle prediction, that is, larger than 1 (YES ofstep S3301 of FIG. 32), a conversion process from the firstintra-color-difference prediction mode to the secondintra-color-difference prediction mode after step S3303 is executed.

When the first intra-color-difference prediction mode IntraPredMode1 issmaller than 16, that is, smaller than or equal to 15 (YES of step S3303of FIG. 32), a value derived by scaling the first intra-color-differenceprediction mode derived by the table of FIG. 14 two times in thevertical direction on the basis of the horizontal prediction(intra-prediction mode 10) and limiting the value to larger than orequal to 2 is set to a value of the second intra-color-differenceprediction mode (steps S3304 to S3307 of FIG. 32). A value obtained bysubtracting 10 showing the horizontal prediction from the firstintra-color-difference prediction mode IntraPredMode1 is set to avariable a (step S3304 of FIG. 32). Next, a value obtained by doublingthe variable a is set to a variable b (step S3305 of FIG. 32). Next, avalue obtained by adding 10 showing the horizontal prediction to b isset to a variable c (step S3306 of FIG. 32). Next, a value obtained bylimiting the value of the variable c to larger than or equal to 2 is setto the second intra-prediction mode IntraPredMode2 (step S3307 of FIG.32) and this derivation process ends. Specifically, when the variable cis smaller than or equal to 2, the value of the intra-prediction modeIntraPredMode2 is set to 2. That is, when a value derived by scaling amode number of the angle prediction of the first intra-color-differenceprediction mode is beyond a range of mode numbers of the angleprediction defined by the intra-prediction mode, the derived value isset to a value in the range. As a result, when an intra-predictionoperation of the color-difference signal at the color-difference format4:2:2 is mounted by hardware, the intra-prediction operation using thesecond intra-color-difference prediction mode can be executed withoutadding the hardware.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is not smaller than 16, that is, larger than or equal to16 (NO of step S3303 of FIG. 32), a value derived by scaling the firstintra-color-difference prediction mode derived by the table of FIG. 14 ½times in the horizontal direction on the basis of the verticalprediction (intra-prediction mode 26) is set to a value of the secondintra-color-difference prediction mode (steps S3308 to S3312 of FIG.32). A value obtained by subtracting 26 showing the vertical predictionfrom the first intra-color-difference prediction mode IntraPredMode1 isset to a variable a (step S3308 of FIG. 32). Next, a value is set to thevariable SignIntraPredMode having a value of −1 when a sign of thevariable a is negative and a value of 1 when the sign is positive or thevariable a is 0 (step S3309 of FIG. 32). Next, a result obtained byexecuting an operation to shift one bit to the right, equivalent to ½times, on an absolute value of the variable a is multiplied with thevariable SignIntraPredMode and an obtained value is set to the variableb (step S3310 of FIG. 32). Also, a result obtained by executing theoperation to shift one bit to the right, equivalent to ½ times, on theabsolute value of the variable a after adding 1 to the absolute valuemay be multiplied with the variable SignIntraPredMode and an obtainedvalue may be set to the variable b. Next, a value obtained by adding 26showing the vertical prediction to b is set to the variable c (stepS3311 of FIG. 32). Next, the value of the variable c is set to thesecond intra-prediction mode IntraPredMode2 (step S3312 of FIG. 32) andthis derivation process ends. In the case in which the value of thevariable c corresponding to the first intra-color-difference predictionmode of 25 becomes 26, the first intra-color-difference prediction modeis set to 25 when 25 is converted into the second intra-color-differenceprediction mode, excluding 26 showing the vertical prediction in whichcoding can be performed at all times. In the case in which the value ofthe variable c corresponding to the first intra-color-differenceprediction mode of 27 becomes 26, the first intra-color-differenceprediction mode is set to 27 when 27 is converted into the secondintra-color-difference prediction mode, excluding 26 showing thevertical prediction in which coding can be performed at all times. Inother words, when the first intra-color-difference prediction mode isconverted into the second intra-color-difference prediction mode usingthe conversion tables illustrated in FIGS. 15, 16, 17, 30, and 33, inthe case in which the first intra-color-difference prediction mode isnot 26 to be the vertical prediction, the value is converted into avalue other than 26 to be the vertical prediction and the secondintra-color-difference prediction mode is derived from the firstintra-color-difference prediction mode. That is, the value is roundedoff (converted) to exclude values (0, 1, 10, 26, and 34) that can bederived from syntax elements (0, 1, 2, and 3 of FIGS. 14 and 25) of theintra-color-difference prediction modes selected when the value of theintra-brightness prediction mode and the value of theintra-color-difference prediction mode are not matched with each other.In this derivation process sequence, to convert the value to exclude avalue 26 that can be derived from a syntax element (1 of FIGS. 14 and25) of the intra-color-difference prediction mode selected when thevalue of the intra-brightness prediction mode and the value of theintra-color-difference prediction mode are not matched with each other,in step S3310, 1 is added to the absolute value of the variable a,before the operation to shift one bit to the right is executed on theabsolute value of the variable a.

In step S3303 of this derivation process sequence, when theintra-color-difference prediction mode IntraPredMode1 is smaller than16, that is, smaller than or equal to 15, a value derived by scaling thefirst intra-color-difference prediction mode two times in the verticaldirection on the basis of the horizontal prediction (intra-predictionmode 10) and limiting the value to larger than or equal to 2 is set to avalue of the second intra-color-difference prediction mode (steps S3304to S3307 of FIG. 32). When the intra-color-difference prediction modeIntraPredMode1 is not smaller than 16, that is, larger than or equal to16, a value derived by scaling the first intra-color-differenceprediction mode ½ times in the horizontal direction on the basis of thevertical prediction (intra-prediction mode 26) is set to a value of thesecond intra-color-difference prediction mode (steps S3308 to S3312 ofFIG. 32). However, when the intra-color-difference prediction modeIntraPredMode1 is smaller than 15, that is, smaller than or equal to 14,a value derived by scaling the first intra-color-difference predictionmode two times in the vertical direction on the basis of the horizontalprediction (intra-prediction mode 10) is set to a value of the secondintra-color-difference prediction mode (steps S3304 to S3307 of FIG.32). When the intra-color-difference prediction mode IntraPredMode1 isnot smaller than 15, that is, larger than or equal to 15, a valuederived by scaling the first intra-color-difference prediction mode ½times in the horizontal direction on the basis of the verticalprediction (intra-prediction mode 26) may be set to a value of thesecond intra-color-difference prediction mode (steps S3308 to S3312 ofFIG. 32) and the conversion result is the same. This is because a valuederived by scaling the first intra-color-difference prediction mode twotimes in the vertical direction on the basis of the horizontalprediction (intra-prediction mode 10) and a value derived by scaling thefirst intra-color-difference prediction mode ½ times in the horizontaldirection on the basis of the vertical prediction (intra-prediction mode26) are equal to each other, when the intra-color-difference predictionmode IntraPredMode1 is 15.

Next, a derivation process sequence when the firstintra-color-difference prediction mode corresponding to the conversiontable of FIG. 33 is converted into the second intra-color-differenceprediction mode at the color-difference format 4:2:2 will be describedusing a flowchart of FIG. 35.

When the prediction is not the angle prediction, that is, the firstintra-prediction mode IntraPredMode1 is 1 or less (NO of step S3401 ofFIG. 35), the value of the first intra-prediction mode IntraPredMode1 isused directly as the second intra-color-difference prediction modeIntraPredMode2 (step S3402 of FIG. 35) and this derivation process ends.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is the angle prediction, that is, larger than 1 (YES ofstep S3401 of FIG. 35), a conversion process from the firstintra-color-difference prediction mode to the secondintra-color-difference prediction mode after step S3403 is executed.

When the first intra-color-difference prediction mode IntraPredMode1 issmaller than 7, that is, smaller than or equal to 6 (YES of step S3403of FIG. 35), a value obtained by subtracting 3 showing the horizontalprediction from the first intra-color-difference prediction modeIntraPredMode1 is set to a variable c (step S3407 of FIG. 35). Next, avalue obtained by limiting the value of the variable c to larger than orequal to 2 is set to the second intra-prediction mode IntraPredMode2(step S3408 of FIG. 35) and this derivation process ends. Specifically,when the variable c is larger than or equal to 2, the variable c is setdirectly to the intra-prediction mode IntraPredMode2 and when thevariable c is smaller than 2, 2 is set to the intra-prediction modeIntraPredMode2. That is, when a value derived by scaling a mode numberof the angle prediction of the first intra-color-difference predictionmode is beyond a range of mode numbers of the angle prediction definedby the intra-prediction mode, the derived value is set to a value in therange. As a result, when an intra-prediction operation of thecolor-difference signal at the color-difference format 4:2:2 is mountedby hardware, the intra-prediction operation using the secondintra-color-difference prediction mode can be executed without addingthe hardware.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is not smaller than 7 and the firstintra-color-difference prediction mode IntraPredMode1 is smaller than14, that is, larger than or equal to 7 and smaller than or equal to 13(NO of step S3403 and YES of step S3404 in FIG. 35), a value derived byscaling the first intra-color-difference prediction mode derived by thetable of FIG. 14 two times in the vertical direction on the basis of thehorizontal prediction (intra-prediction mode 10) is set to a value ofthe second intra-color-difference prediction mode (steps S3409 to S3412of FIG. 35). A value obtained by subtracting 10 showing the horizontalprediction from the first intra-color-difference prediction modeIntraPredMode1 is set to a variable a (step S3409 of FIG. 35). Next, avalue obtained by doubling the variable a is set to the variable b (stepS3410 of FIG. 35). Next, a value obtained by adding 10 showing thehorizontal prediction to b is set to the variable c (step S3411 of FIG.35). Next, the value of the variable c is set to the secondintra-prediction mode IntraPredMode2 (step S3412 of FIG. 35) and thisderivation process ends.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is not smaller than 14 and is smaller than 21, that is,larger than or equal to 14 and smaller than or equal to 20 (NO of stepS3404 and YES of step S3405 in FIG. 35), a value obtained by adding 3 tothe first intra-color-difference prediction mode IntraPredMode1 is setto the variable c (step S3413 of FIG. 35). Next, the value of thevariable c is set to the second intra-prediction mode IntraPredMode2(step S3414 of FIG. 35) and this derivation process ends.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is not smaller than 21 and is smaller than 32, that is,larger than or equal to 21 and smaller than or equal to 31 (NO of stepS3405 and YES of step S3406 in FIG. 35), a value derived by scaling thefirst intra-color-difference prediction mode derived by the table ofFIG. 14 ½ times in the horizontal direction on the basis of the verticalprediction (intra-prediction mode 26) is set to a value of the secondintra-color-difference prediction mode (steps S3415 to S3419 of FIG.35). A value obtained by subtracting 26 showing the vertical predictionfrom the first intra-color-difference prediction mode IntraPredMode1 isset to a variable a (step S3415 of FIG. 35). Next, a value is set to thevariable SignIntraPredMode having a value of −1 when a sign of thevariable a is negative and a value of 1 when the sign is positive or thevariable a is 0 (step S3416 of FIG. 35). Next, a result obtained byexecuting an operation to shift one bit to the right, equivalent to ½times, on an absolute value of the variable a is multiplied with thevariable SignIntraPredMode and an obtained value is set to the variableb (step S3417 of FIG. 35). Also, a result obtained by executing theoperation to shift one bit to the right, equivalent to ½ times, on theabsolute value of the variable a after adding 1 to the absolute valuemay be multiplied with the variable SignIntraPredMode and an obtainedvalue may be set to the variable b. Next, a value obtained by adding 26showing the vertical prediction to b is set to the variable c (stepS3418 of FIG. 35) Next, the value of the variable c is set to the secondintra-prediction mode IntraPredMode2 (step S3419 of FIG. 35) and thisderivation process ends. In the case in which the value of the variablec corresponding to the first intra-color-difference prediction mode of25 becomes 26, the first intra-color-difference prediction mode is setto 25 when 25 is converted into the second intra-color-differenceprediction mode, excluding 26 showing the vertical prediction in whichcoding can be performed at all times. In the case in which the value ofthe variable c corresponding to the first intra-color-differenceprediction mode of 27 becomes 26, the first intra-color-differenceprediction mode is set to 27 when 27 is converted into the secondintra-color-difference prediction mode, excluding 26 showing thevertical prediction in which coding can be performed at all times. Thatis, the value is rounded off (converted) to exclude values (0, 1, 10,26, and 34) that can be derived from syntax elements (0, 1, 2, and 3 ofFIGS. 14 and 25) of the intra-color-difference prediction modes selectedwhen the value of the intra-brightness prediction mode and the value ofthe intra-color-difference prediction mode are not matched with eachother. In this derivation process sequence, to convert the value toexclude a value 26 that can be derived from a syntax element (1 of FIGS.14 and 25) of the intra-color-difference prediction mode selected whenthe value of the intra-brightness prediction mode and the value of theintra-color-difference prediction mode are not matched with each other,in step S3417, 1 is added to the absolute value of the variable a,before the operation to shift one bit to the right is executed on theabsolute value of the variable a.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is not smaller than 32, that is, larger than or equal to32 (NO of step S3416 of FIG. 35), a value derived by subtracting 3 fromthe first intra-color-difference prediction mode IntraPredMode1 is setto the variable c (step S3420 of FIG. 35). Next, the value of thevariable c is set to the second intra-prediction mode IntraPredMode2(step S3421 of FIG. 35) and this derivation process ends.

In step S3403 of this derivation process sequence, when theintra-color-difference prediction mode IntraPredMode1 is smaller than orequal to 6, a value obtained by limiting a value obtained by subtracting3 from the first intra-color-difference prediction mode to larger thanor equal to 2 is set to the second intra-color-difference predictionmode. However, even when the condition determination of step S3403 isomitted for the simplification and the intra-color-difference predictionmode IntraPredMode1 is smaller than or equal to 6, similarly to when theintra-color-difference prediction mode IntraPredMode1 is larger than orequal to 7 and smaller than or equal to 13, a value derived by scalingthe first intra-color-difference prediction mode two times in thevertical direction on the basis of the horizontal prediction(intra-prediction mode 10) can be set to a value of the secondintra-color-difference prediction mode (steps S3409 to S3412 of FIG.35). However, in step S3412, a value obtained by limiting the variable cderived by step S3411 to larger than or equal to 2 is set to the secondintra-color-difference prediction mode, similarly to S3408.

In addition, in step S3406 of this derivation process sequence, when theintra-color-difference prediction mode IntraPredMode1 is larger than orequal to 32, a value obtained by subtracting 3 from the firstintra-color-difference prediction mode is set to the secondintra-color-difference prediction mode. However, even when the conditiondetermination of step S3406 is omitted for the simplification and theintra-color-difference prediction mode IntraPredMode1 is larger than orequal to 32, similarly to when the intra-color-difference predictionmode IntraPredMode1 is larger than or equal to 21 and smaller than orequal to 31, a value derived by scaling the first intra-color-differenceprediction mode ½ times in the horizontal direction on the basis of thevertical prediction (intra-prediction mode 26) can be set to a value ofthe second intra-color-difference prediction mode (steps S3415 to S3419of FIG. 35).

Meanwhile, when the color-difference format is 4:2:0 or 4:4:4, becausethe sampling ratios of the intra-prediction direction of the brightnesssignal and the horizontal direction and the vertical direction of thecolor-difference signal are matched with each other, it is not necessaryto convert the first intra-color-difference prediction mode derived bythe table of FIG. 14 into the second intra-color-difference predictionmode. This will be described with reference to FIGS. 22A and 22B. FIGS.22A and 22B are a diagram illustrating a correspondence relation ofprediction directions of intra-predictions of a brightness signal and acolor-difference signal when the color-difference format is 4:2:0. FIG.22A illustrates an arrangement of the brightness signal and thecolor-difference signal when the color-difference format is 4:2:0 and acolor-difference signal is sampled at a density of ½ in both horizontaland vertical directions, with respect to a brightness signal, and aspectratios of pixels of the brightness signal and the color-differencesignal are equal to each other. An arrow extended from a pixel P4 to apixel P5, shown by a symbol 2704, shows an intra-prediction direction ofthe pixel P4 of the brightness signal. An arrow extended from a pixel P1to a pixel P2, shown by a symbol 2705, shows an intra-predictiondirection of the pixel P1 of the color-difference signal. The arrowextended from the pixel P4 to the pixel P5, shown by the symbol 2704,and the arrow extended from the pixel P1 to the pixel P2, shown by thesymbol 2705, extend in the same direction and the intra-predictiondirections are the same. In this case, in an array of thecolor-difference signals illustrated in FIG. 22B, the intra-predictiondirection of the brightness signal is the intra-prediction direction ofthe color-difference signal as shown by a symbol 2706 and the pixel P2of the reference destination of the pixel P1 of the color-differencesignal can be surely referred to.

When the value of the intra-color-difference prediction mode ispredicted in the intra-predictor 103 in consideration of the pointsdescribed above, a value of the intra-color-difference prediction modeis predicted from a value of the intra-brightness prediction mode of theprediction block of the same position as the prediction block of thecolor-difference signal, according to the color-difference format. Thatis, when the value of the intra-color-difference prediction mode ispredicted at the color-difference format 4:2:0 or 4:4:4 in which aspectratios of the pixels of the brightness signal and the color-differencesignal are the same, the value of the intra-color-difference predictionmode obtained from the table of FIG. 14 is used directly as the value ofthe intra-color-difference prediction mode for the color-differenceformat 4:2:0 or 4:4:4 and the intra-prediction of the color-differencesignal is performed according to the intra-color-difference predictionmode. When the value of the intra-color-difference prediction mode ispredicted at the color-difference format 4:2:2, a value of theintra-color-difference prediction mode for the color-difference format4:2:2 is derived from the value of the intra-color-difference predictionmode obtained from the table of FIG. 14 by the conversion tableillustrated in FIG. 15, 16, 17, 30, or 33 and the intra-prediction ofthe color-difference signal is performed according to theintra-color-difference prediction mode.

FIG. 25 is a table to derive a value of a syntax elementintra_chroma_pred_mode[x0][y0] regarding an intra-color-differenceprediction mode from a value of an intra-color-difference predictionmode and a value of an intra-brightness prediction mode of a predictionblock of a brightness signal of the same position as a prediction blockof a color-difference signal and the table of FIG. 25 used at the codingside corresponds to the table of FIG. 14 used at the decoding side. Atthe coding side, a value of the syntax elementintra_chroma_pred_mode[x0][y0] is derived using the table illustrated inFIG. 25.

In the case in which the value of the first or secondintra-color-difference prediction mode is 0, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not0, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 0 and if the value of the intra-brightness prediction mode is0, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 4.

In the case in which the value of the first or secondintra-color-difference prediction mode is 26, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not26, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 1 and if the value of the intra-brightness prediction mode is26, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 4.

In the case in which the value of the first or secondintra-color-difference prediction mode is 10, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not10, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 2 and if the value of the intra-brightness prediction mode is10, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 4.

In the case in which the value of the first or secondintra-color-difference prediction mode is 1, if the value of theintra-brightness prediction mode of the prediction block of the sameposition as the prediction block of the color-difference signal is not10, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 3 and if the value of the intra-brightness prediction mode is10, the value of the syntax element intra_chroma_pred_mode[x0][y0] takesa value of 4.

In the case in which the value of the first intra-color-differenceprediction mode is 34, if the value of the intra-brightness predictionmode of the prediction block of the same position as the predictionblock of the color-difference signal is not 0, the value of the syntaxelement intra_chroma_pred_mode[x0][y0] takes a value of 0, if the valueof the intra-brightness prediction mode is 1, the value of the syntaxelement intra_chroma_pred_mode[x0][y0] takes a value of 1, if the valueof the intra-brightness prediction mode is 2, the value of the syntaxelement intra_chroma_pred_mode[x0][y0] takes a value of 2, if the valueof the intra-brightness prediction mode is 3, the value of the syntaxelement intra_chroma_pred_mode[x0][y0] takes a value of 3, and if thevalue of the intra-brightness prediction mode is 34, the value of thesyntax element intra_chroma_pred_mode[x0][y0] takes a value of 4.

When the value of the first intra-color-difference prediction mode isequal to the value of the intra-brightness prediction mode of theprediction block of the same position as the prediction block of thecolor-difference signal, the value of the syntax elementintra_chroma_pred_mode[x0][y0] takes a value of 4. However, when thevalue of the intra-color-difference prediction mode is predicted at thecolor-difference format 4:2:2, in the intra-predictor 103, the secondintra-color-difference prediction mode for the color-difference format4:2:2 is derived from the first intra-color-difference prediction modefor the color-difference format 4:2:0 or 4:4:4 by the conversion tableillustrated in FIG. 15, 16, 17, 30, or 33. The secondintra-color-difference prediction mode is used for the intra-predictionof the color-difference signal for the color-difference format 4:2:2.

When the prediction block of the same position as the prediction blockof the color-difference signal is specified, the prediction block may bespecified by referring to a division index PartIdx to specify eachprediction block and may be specified by referring to the coordinatesshowing a position of each prediction block.

The number of intra-color-difference prediction modes of the predictionblock in the coding block is different according to a combination of adivision mode and a color-difference format supplied from thecolor-difference format setter 101. When the division mode is 2N×2Ndivision, a value of a syntax element regarding anintra-color-difference prediction mode of one prediction block isderived for each coding block, regardless of a type of thecolor-difference format.

When the division mode is N×N division and the color-difference formatis 4:2:0, a value of a syntax element regarding anintra-color-difference prediction mode of one prediction block isderived for each coding block. When the division mode is the N×Ndivision and the color-difference format is 4:2:2, values of syntaxelements regarding intra-color-difference prediction modes of twoprediction blocks are derived for each coding block. When the divisionmode is the N×N division and the color-difference format is 4:4:4,values of syntax elements regarding intra-color-difference predictionmodes of four prediction blocks are derived for each coding block. FIG.11A to 11D is a diagram illustrating a method of dividing acolor-difference signal of a coding block in the N×N division when theintra-prediction is performed into a prediction block. FIG. 11Aillustrates a brightness signal in the N×N division, FIG. 11Billustrates a color-difference signal in the N×N division when thecolor-difference format is 4:2:0, FIG. 11C illustrates acolor-difference signal in the N×N division when the color-differenceformat is 4:2:2, and FIG. 11D illustrates a color-difference signal inthe N×N division when the color-difference format is 4:4:4. When thecolor-difference formats are 4:2:0 and 4:4:4, a coding block of thebrightness signal and a coding block of the color-difference signal aresimilar to each other and aspect ratios of both blocks are matched witheach other. When the color-difference format is 4:2:2, a coding block ofthe brightness signal and a coding block of the color-difference signalare not similar to each other and aspect ratios of both coding blocksare different from each other. Similarly to when the color-differenceformat is 4:2:0, when the color-difference format is 4:2:2 or 4:4:4, acoding block can be used as one prediction block without dividing thecoding block, in a color-difference signal in which the division mode isthe N×N division. Similarly to when the color-difference format is4:0:0, when the color-difference format is 4:2:2 or 4:4:4, a codingblock can be used as one prediction block without dividing the codingblock, in a color-difference signal in which the division mode is theN×N division.

When a prediction mode (PredMode) of a coding block is inter-prediction(MODE_INTER), the syntax element deriver 124 regarding theinter-prediction information derives a value of a syntax elementregarding inter-prediction information of a prediction block unit andsupplies the derived value of each syntax element to the entropy coder126. The inter-prediction information of the prediction block unitincludes information such as inter-prediction modes (L0 prediction, L1prediction, and both predictions), indexes to specify a plurality ofreference pictures, and a motion vector.

The entropy coder 126 performs entropy coding on a value of a syntaxelement regarding the coding information of the coding block unitsupplied from the syntax element deriver 121 regarding the codinginformation of the coding block unit, a value of a syntax elementregarding the intra-brightness prediction mode of the prediction blockof the brightness signal supplied from the syntax element deriver 122regarding the intra-brightness prediction mode, a value of a syntaxelement regarding the intra-color-difference prediction mode of theprediction block of the color-difference signal supplied from the syntaxelement deriver 123 regarding the intra-color-difference predictionmode, and a value of a syntax element regarding the intra-predictioninformation of the prediction block unit supplied from the syntaxelement deriver 124 regarding the inter-prediction information,according to a prescribed syntax rule. At this time, theintra-prediction mode coding controller 125 controls order of entropycoding of the intra-brightness prediction mode and theintra-color-difference prediction mode, according to the division modeand the color-difference format, and the entropy coder 126 executes anentropy coding process of the intra-brightness prediction mode and theintra-color-difference prediction mode, in order commanded by theintra-prediction mode coding controller 125.

Next, a decoding process of coding information of the coding block andprediction block units executed by the second bitstream decoder 203 ofFIG. 2 will be described on the basis of a point relating to theintra-prediction mode to be a characteristic of the embodiment. FIG. 13is a block diagram illustrating a configuration of the second bitstreamdecoder 203 of FIG. 2.

As illustrated in FIG. 13, the second bitstream decoder 203 of FIG. 2includes an intra-prediction mode decoding controller 221, an entropydecoder 222, a coding information deriver 223 of a coding block unit, anintra-brightness prediction mode deriver 224, an intra-color-differenceprediction mode deriver 225, and an inter-prediction information deriver226. In individual elements configuring the second bitstream decoder203, a process according to color-difference format information suppliedfrom the color-difference format manager 205 is executed and a processaccording to coding information such as a prediction mode and a divisionmode of a coding block unit is executed.

The entropy decoder 222 performs entropy decoding on the secondbitstream including the coding information of the coding block andprediction block units supplied from the bitstream separator 201according to the prescribed syntax rule and obtains the value of thesyntax element regarding the coding information of the coding blockunit, the value of the syntax element regarding the intra-brightnessprediction mode of the prediction block of the brightness signal, thevalue of the syntax element regarding the intra-color-differenceprediction mode of the prediction block of the color-difference signal,and the value of the syntax element regarding the inter-predictioninformation of the prediction block unit. At this time, theintra-prediction mode decoding controller 221 controls order of theentropy decoding of the intra-brightness prediction mode and theintra-color-difference prediction mode according to the division modeand the color-difference format and the entropy decoder 222 executes anentropy decoding process of the intra-brightness prediction mode and theintra-color-difference prediction mode in order commanded by theintra-prediction mode decoding controller 221. The intra-prediction modedecoding controller 221 is a controller corresponding to theintra-prediction mode coding controller 125 of the coding side. Theintra-prediction mode decoding controller 221 sets decoding order of theintra-prediction mode equal to the coding order of the intra-predictionmode set by the intra-prediction mode coding controller 125 according tothe division mode and the color-difference format and controls thedecoding order of the intra-prediction mode of the entropy decoder 222.The entropy decoder 222 is a decoder corresponding to the entropy coder126 of the coding side and executes an entropy decoding processaccording to the same rule as the syntax rule used by the entropy coder126.

The value of the syntax element regarding the coding information of thecoding block unit obtained by decoding is supplied to the codinginformation deriver 223 of the coding block unit, the value of thesyntax element regarding the intra-brightness prediction mode of theprediction block of the brightness signal is supplied to theintra-brightness prediction mode deriver 224, the value of the syntaxelement regarding the intra-color-difference prediction mode of theprediction block of the color-difference signal is supplied to theintra-color-difference prediction mode deriver 225, and the value of thesyntax element regarding the inter-prediction information of theprediction block unit is supplied to the inter-prediction informationderiver 226.

The coding information deriver 223 of the coding block unit derives thecoding information of the coding block unit from the supplied value ofthe syntax element regarding the coding information of the coding blockunit and supplies the coding information to the intra-predictor 206 orthe inter-predictor 207 through the switch 212.

The coding information deriver 223 of the coding block unit is a codinginformation deriver corresponding to the syntax element deriver 121regarding the coding information of the coding block unit of the codingside and derives coding information according to a common rule at thecoding side and the decoding side. Values regarding a prediction mode(PredMode) to determine intra-prediction (MODE_INTRA) orinter-prediction (MODE_INTER) of the coding block and a division mode(PartMode) to determine a shape of a prediction block are derived by thecoding information deriver 223 of the coding block unit.

When the prediction mode (PredMode) of the coding block derived by thecoding information deriver 223 of the coding block unit is theintra-prediction (MODE_INTRA), the intra-brightness prediction modederiver 224 derives the intra-brightness prediction mode of theprediction block of the brightness signal from the supplied value of thesyntax element regarding the intra-brightness prediction mode of theprediction block of the brightness signal, supplies the derived value tothe intra-color-difference prediction mode deriver 225, and supplies thederived value to the intra-predictor 206 through the switch 212. Theintra-brightness prediction mode deriver 224 is a deriver correspondingto the syntax element deriver 122 regarding the intra-brightnessprediction mode of the coding side and derives the intra-brightnessprediction mode according to the common rule at the coding side and thedecoding side. The syntax elements regarding the intra-brightnessprediction mode are a syntax element prev_intra_luma_pred_flag[x0][y0]to be a flag showing whether prediction can be performed from anintra-brightness prediction mode of a neighboring block, a syntaxelement mpm_idx[x0][y0] to be an index showing a prediction block of aprediction origin, and a syntax element rem_intra_luma_pred_mode[x0][y0]showing an intra-brightness prediction mode of a prediction block unit.When the intra-brightness prediction mode is derived, a correlation withan intra-brightness prediction mode of a neighboring block stored in thecoding information memory 210 is used. When prediction can be performedfrom the intra-brightness prediction mode of the neighboring block, asyntax element prev_intra_luma_pred_flag[x0][y0] to be a flag showingthat the value is used become 1 (true) and the intra-brightnessprediction mode of the neighboring prediction block shown by the syntaxelement mpm_idx[x0][y0] to be the syntax showing the prediction block ofthe prediction origin is set to the intra-brightness prediction mode ofthe prediction mode. When the syntax elementprev_intra_luma_pred_flag[x0][y0] is 0 (false), the intra-brightnessprediction mode is not predicted from the neighboring prediction blockand the intra-brightness prediction mode is derived from the value ofthe syntax element rem_intra_luma_pred_mode[x0][y0] showing the decodedintra-brightness prediction mode.

The number of intra-brightness prediction modes of the prediction blockin the coding block is different according to the division mode. Whenthe division mode is 2N×2N division, values of intra-brightnessprediction modes of one set of prediction blocks are derived for eachcoding block and when the division mode is N×N division, values ofintra-brightness prediction modes of four sets of prediction blocks arederived for each coding block.

When the prediction mode (PredMode) of the coding block derived by thecoding information deriver 223 of the coding block unit is theintra-prediction (MODE_INTRA), the intra-color-difference predictionmode deriver 225 derives a value of the first intra-color-differenceprediction mode from the supplied value of the syntax elementintra_chroma_pred_mode[x0][y0] regarding the intra-color-differenceprediction mode of the prediction block of the color-difference signaland the value of the intra-brightness prediction mode supplied from theintra-brightness prediction mode deriver 224, according to the table ofFIG. 14. When the color-difference format is 4:2:0 or 4:4:4, the firstintra-color-difference prediction mode is supplied as theintra-prediction mode of the color-difference signal to theintra-predictor 206 through the switch 212. In addition, when thecolor-difference format is 4:2:2, the second intra-color-differenceprediction mode is derived from the first intra-color-differenceprediction mode by the conversion table of FIG. 15, 16, 17, 30, or 33and the second intra-color-difference prediction mode is supplied as theintra-prediction mode of the color-difference signal to theintra-predictor 206 through the switch 212. The intra-color-differenceprediction mode deriver 225 is a coding information derivercorresponding to the syntax element deriver 123 regarding theintra-color-difference prediction mode of the coding side and derivesthe intra-color-difference prediction mode according to the common ruleat the coding side and the decoding side. At the coding side, in codingof the intra-color-difference prediction mode, a correlation with anintra-brightness prediction mode of a prediction block of a brightnesssignal of the same position as the prediction block of thecolor-difference signal is used. At the coding side, when it isdetermined that a prediction value from the intra-brightness predictionmode of the prediction block of the brightness signal of the sameposition as the prediction block of the color-difference signal is mostsuitable, a value of the intra-color-difference prediction mode ispredicted from the value of the intra-brightness prediction mode. Whenit is determined that setting an independent value to theintra-color-difference prediction mode is more preferable than theprediction from the intra-brightness prediction mode, a mechanism forsetting any value of 0 (plane prediction), 1 (average value prediction),10 (horizontal prediction), 26 (vertical prediction), and 34 (obliqueprediction) to be representative intra-prediction modes to theintra-color-difference prediction mode is used and a code amount isreduced.

When the prediction mode (PredMode) of the coding block is theintra-prediction (MODE_INTER), the inter-prediction information deriver226 derives the inter-prediction information from the value of thesyntax element regarding the inter-prediction information of theprediction block unit and supplies the derived value of theinter-prediction information to the inter-predictor 207 through theswitch 212. The inter-prediction information deriver 226 is aninter-prediction information deriver corresponding to the syntax elementderiver 124 regarding the inter-prediction information of the codingside and derives the inter-prediction information according to thecommon rule at the coding side and het decoding side. The derivedinter-prediction information of the prediction block unit includesinformation such as inter-prediction modes (L0 prediction, L1prediction, and both predictions), indexes to specify a plurality ofreference pictures, and a motion vector.

Next, a process sequence of decoding of the inter-prediction mode andthe intra-prediction at the decoding side will be described. FIG. 26 isa diagram illustrating a process sequence of decoding of theintra-prediction mode and the intra-prediction executed by the secondbitstream decoder 203 and the intra-predictor 206 of the decoding side.First, the intra-brightness prediction mode is decoded by theintra-brightness prediction mode deriver 224 of the second bitstreamdecoder 203 (step S4001 of FIG. 26). Next, the firstintra-color-difference prediction mode is decoded by theintra-color-difference prediction mode deriver 225 of the secondbitstream decoder 203 according to the table of FIG. 14 (step S4002 ofFIG. 26). Next, when the color-difference format is not 4:2:2 (NO ofstep S4003 of FIG. 26), the process proceeds to step S4004 and when thecolor-difference format is 4:2:2 (YES of step S4003 of FIG. 26), thesecond intra-color-difference prediction mode is derived from the firstintra-color-difference prediction mode by the intra-color-differenceprediction mode deriver 225 of the second bitstream decoder 203, usingthe conversion table of FIG. 15, 16, 17, 30, or 33 (step S4004 of FIG.26). Next, the intra-prediction of the brightness signal and thecolor-difference signal is performed by the intra-predictor 206 (stepS4004 of FIG. 26). A process for deriving the secondintra-color-difference prediction mode from the firstintra-color-difference prediction mode in step S4004 of FIG. 26 may beexecuted by the intra-predictor 206, instead of theintra-color-difference prediction mode deriver 225 of the secondbitstream decoder 203.

When the intra-prediction mode of the plane prediction is 0 and theintra-prediction mode of the average value prediction is 1, similarly tothe cases of the color-difference formats 4:2:0 and 4:4:4, in the caseof the color-difference format 4:2:2, the intra-prediction mode of theplane prediction is set to 0, the intra-prediction mode of the averagevalue prediction is set to 1, and the intra-prediction is executed. Forthis reason, in the conversion table of FIG. 15, 16, 17, 30, or 33, eventhough the first intra-color-difference prediction mode is convertedinto the second intra-color-difference prediction mode, the values arethe same. Therefore, in the intra-prediction modes 0 and 1 not to be theangle prediction, the intra-prediction may be executed after the valueof the second intra-color-difference prediction mode is derived from thevalue of the first intra-color-difference prediction mode using theconversion table of FIG. 15, 16, 17, 30, or 33 and the intra-predictionmay be executed according to the first intra-color-difference predictionmode without deriving the second intra-color-difference prediction modeusing the conversion table of FIG. 15, 16, 17, 30, or 33.

In the picture coding device and the picture decoding device accordingto this embodiment, when the color-difference format is 4:2:2, the valueof the second intra-color-difference prediction mode is derived from thevalue of the first intra-color-difference prediction mode using theconversion table of FIG. 15, 16, 17, 30, or 33. However, the value ofthe second intra-color-difference prediction mode may be derived fromthe value of the first intra-color-difference prediction mode using acalculation formula, instead of the conversion table.

In the picture coding device and the picture decoding device accordingto this embodiment, in the case of the color-difference format 4:2:2 inwhich the aspect ratios of the pixels of the brightness signal and thecolor-difference signal are different from each other, the value of thesecond intra-color-difference prediction mode is derived from the valueof the first intra-color-difference prediction mode using the conversiontable of FIG. 15, 16, 17, 30, or 33. However, in the intra-predictor 103of the coding device and the intra-predictor 206 of the decoding device,instead of converting the first intra-color-difference prediction modeinto the second intra-color-difference prediction mode, a tableassociated with the angle of the intra-prediction for thecolor-difference signal of the color-difference format 4:2:2 in additionto the angle (the angle of the intra-prediction for the brightnesssignal and the angle of the intra-prediction for the color-differencesignals of the color-difference formats 4:2:0 and 4:4:4) of theintra-prediction for the signal other than the color-difference signalof the color-difference format 4:2:2 from the intra-prediction modeillustrated in FIG. 27 or 28 may be prepared, the angle of theintra-prediction for the color-difference signal of the color-differenceformat 4:2:2 may be derived from the first intra-color-differenceprediction mode using the table, when the color-difference format is4:2:2, and the intra-prediction of the color-difference signal may beperformed using the angle. FIGS. 27 and 28 are tables used when an angleof the intra-prediction for the color-difference signal of thecolor-difference format 4:2:2 is derived from the intra-prediction mode,in addition to the angle (the angle of the intra-prediction for thebrightness signal and the angle of the intra-prediction for thecolor-difference signals of the color-difference formats 4:2:0 and4:4:4) of the intra-prediction for the signal other than thecolor-difference signal of the color-difference format 4:2:2. FIG. 27 isa table in which the angle of the intra-prediction for thecolor-difference signal of the color-difference format 4:2:2 is set toobtain the same result as when the value of the secondintra-color-difference prediction mode is derived from the value of thefirst intra-color-difference prediction mode using the conversion tableof FIG. 15 and the intra-prediction of the color-difference signal isperformed. When the color-difference format is 4:2:2, the angle of theintra-prediction for the color-difference signal is derived according tothe table of FIG. 27 and the intra-prediction is performed, so that thesame result as when the value of the second intra-color-differenceprediction mode is derived from the value of the firstintra-color-difference prediction mode using the conversion table ofFIG. 15 and the intra-prediction of the color-difference signal isperformed is obtained.

FIG. 28 is a table in which a result obtained by multiplying the angleof the intra-prediction with 2 in the vertical direction and ½ in thehorizontal direction is set. A derivation process sequence of the tableof FIG. 28 when the angle of the first intra-color-difference predictionis converted into the angle of the second intra-color-differenceprediction at the color-difference format 4:2:2 will be described usinga flowchart of FIG. 29.

In each value of the first intra-prediction mode IntraPredMode1 from 0to 34, an angle IntraPredMode2 of the second intra-prediction used forthe intra-prediction of the color-difference signal of thecolor-difference format 4:2:2 is derived by a sequence of the flowchartof FIG. 29.

First, when the prediction is not the angle prediction, that is, thefirst intra-prediction mode IntraPredMode1 is smaller than or equal to 1(NO of step S3201 of FIG. 29), this derivation process ends. When thefirst intra-prediction mode IntraPredMode1 is smaller than or equal to1, this corresponds to plane prediction (intra-prediction modeintraPredMode1=0) in which prediction is performed by interpolating apixel value from a surrounding decoded block and average valueprediction (intra-prediction mode intraPredMode1=1) in which predictionis performed by deriving an average value from a surrounding decodedblock.

Meanwhile, when the first intra-color-difference prediction modeIntraPredMode1 is the angle prediction, that is, larger than 1 (YES ofstep S3201 of FIG. 29), a conversion process from the angle of the firstintra-prediction to the angle of the second intra-prediction after stepS3202 is executed.

When the intra-color-difference prediction mode IntraPredMode1 issmaller than 18 (YES of step S3202 of FIG. 29), an angle IntraPredAngle1of the first intra-prediction corresponding to the firstintra-prediction mode IntraPredMode1 is doubled and is set to an angleIntraPredAngle2 of the second intra-prediction (step S3203 of FIG. 29).In addition, the angle IntraPredAngle2 of the second intra-prediction islimited to larger than or equal to −32 and smaller than or equal to 32(step S3204 of FIG. 29) and this derivation process ends. Specifically,when the angle IntraPredAngle2 of the second intra-prediction is smallerthan −32, −32 is set to the angle IntraPredAngle2 of the secondintra-prediction and when the angle IntraPredAngle2 of the secondintra-prediction is larger than 32, 32 is set to the angleIntraPredAngle2 of the second intra-prediction. When theintra-prediction mode IntraPredMode2 is larger than or equal to 2 and issmaller than 18, the intra-prediction mode IntraPredMode2 is maintained.

Meanwhile, when the intra-color-difference prediction modeIntraPredMode1 is not smaller than 18, that is, larger than or equal to18 (NO of step S3202 of FIG. 29), the angle IntraPredAngle1 of the firstintra-prediction corresponding to the first intra-color-differenceprediction mode IntraPredMode1 is multiplied with ½ and is set to theangle IntraPredAngle2 of the second intra-prediction (step S3205 of FIG.29) and this derivation process ends. In this embodiment, a resultobtained by executing an operation to shift one bit to the right,equivalent to ½ times, on the angle IntraPredAngle1 of the firstintra-prediction is set to the angle IntraPredAngle2 of the secondintra-prediction.

In the above description, when the color-difference format is 4:2:2, theangle of the second intra-prediction is derived from the angle of thefirst intra-prediction using the table of FIG. 28. However, in theintra-predictor 103 of the coding device and the intra-predictor 206 ofthe decoding device, the angle of the second intra-prediction may bederived from the value of the first intra-color-difference predictionmode, by a derivation method according to the process sequence of FIG.29, instead of the table of FIG. 28.

The bitstream of the moving pictures output by the picture coding deviceaccording to the embodiment described above has the specific data formatto be decoded according to the coding method used in the embodiment andthe picture decoding device corresponding to the picture coding devicecan decode the bitstream of the specific data format.

When a wired or wireless network is used to exchange the bitstreambetween the picture coding device and the picture decoding device, adata format of the bitstream may be converted into a data formatsuitable for a transmission form of a communication path and thebitstream may be transmitted. In this case, a picture transmittingdevice to convert the bitstream output by the picture coding device intocoding data of the data format suitable for the transmission form of thecommunication path and transmit the coding data to a network and apicture receiving device to receive the coding data from the network,restore the coding data to the bitstream, and supply the bitstream tothe picture decoding device are provided.

The picture transmitting device includes a memory that buffers thebitstream output by the picture coding device, a packet processor thatpacketizes the bitstream, and a transmitter that transmits thepacketized coding data through the network. The picture receiving deviceincludes a receiver that receives the packetized coding data through thenetwork, a memory that buffers the received coding data, and a packetprocessor that processes the coding data to generate a bitstream andprovides the bitstream to the picture decoding device.

The processes regarding the coding and the decoding can be realized astransmission, accumulation, and reception devices using hardware and canbe realized by firmware stored in a read only memory (ROM) and a flashmemory or software such as a computer. A firmware program and a softwareprogram can be recorded on a recording medium readable by the computerand can be provided, the firmware program and the software program canbe provided from a server through the wired or wireless network, and thefirmware program and the software program can be provided as databroadcasting of terrestrial or satellite digital broadcasting.

The present invention has been described on the basis of the embodiment.However, the embodiment is only exemplary and it should be understood bythose skilled in the art that various modifications can be made in acombination of components and processes and the modifications can beincluded in a range of the present invention.

[Item 1]

A picture coding device for coding information regarding anintra-prediction mode in a prediction block unit and coding picturesignals including a brightness signal and a color-difference signal in aconversion block unit using intra-prediction, comprising:

an intra-brightness prediction mode coder that sets a prediction blockof the brightness signal, codes a syntax element regarding theintra-prediction mode of the brightness signal, on the basis of theintra-prediction mode of the brightness signal showing anintra-prediction method of the prediction block of the bright signal,and codes information regarding an intra-brightness prediction mode in abitstream;

an intra-color-difference prediction mode coder that sets a predictionblock of the color-difference signal, codes a syntax element regardingan intra-color-difference prediction mode of the color-difference signalby referring to the intra-brightness prediction mode, on the basis ofthe intra-color-difference prediction mode of the color-differencesignal showing an intra-prediction method of the prediction block of thecolor-difference signal, and codes information regarding theintra-color-difference prediction mode in the bitstream;

a brightness signal intra-predictor that predicts a brightness signal ofa conversion block of the brightness signal from a surroundingbrightness signal of the conversion block of the brightness signal,according to the intra-brightness prediction mode; and

a color-difference signal intra-predictor that predicts acolor-difference signal of a conversion block of the color-differencesignal from a surrounding color-difference signal of the conversionblock of the color-difference signal, according to theintra-color-difference prediction mode.

[Item 2]

The picture coding device according to item 1, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, theintra-color-difference prediction mode coder converts a mode number of afirst intra-color-difference prediction mode used when the aspect ratiosare equal to each other into a scaled mode number and derives a secondintra-color-difference prediction mode used when the aspect ratios aredifferent from each other.

[Item 3]

The picture coding device according to item 1, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, thecolor-difference signal intra-predictor scales an angle of a predictiondirection defined by a first intra-color-difference prediction mode usedwhen the aspect ratios are equal to each other, specifies a mode numberof an intra-prediction mode of an angle close to the scaled angle, andderives a second intra-prediction mode used when the aspect ratios aredifferent from each other.

[Item 4]

The picture coding device according to item 1, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, thecolor-difference signal intra-predictor refers to a conversion table inwhich a mode number of an intra-color-difference prediction mode usedwhen the aspect ratios are equal to each other and an angle afterscaling are associated with each other and performs the intra-predictionon the color-difference signal using a converted angle.

[Item 5]

The picture coding device according to item 1, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, theintra-color-difference prediction mode coder derives a secondintra-color-difference prediction mode used when the aspect ratios aredifferent from each other, on the basis of a conversion table in which avalue close to a prediction direction derived by scaling an angle of aprediction direction corresponding to a first intra-color-differenceprediction mode used when the aspect ratios are equal to each other isset.

[Item 6]

The picture coding device according to item 5, wherein

when a value of the second intra-color-difference prediction modederived by scaling the angle of the prediction direction of the firstintra-color-difference prediction mode is beyond a range of mode numbersdefined by an intra-prediction mode, the intra-color-differenceprediction mode coder sets the value of the secondintra-color-difference prediction mode to a value in the range of themode numbers defined by the intra-prediction mode.

[Item 7]

The picture coding device according to item 5 or 6, wherein

the intra-color-difference prediction mode coder refers to a syntaxelement regarding an intra-color-difference prediction mode and acorresponding intra-brightness prediction mode and specifies a modenumber of the first intra-color-difference prediction mode, and

the color-difference signal intra-predictor derives the secondintra-color-difference prediction mode, on the basis of a conversiontable in which a value obtained by scaling the mode number of the firstintra-color-difference prediction mode is set to exclude a mode numberderived when values of the intra-brightness prediction mode and thefirst intra-color-difference prediction mode are not matched with eachother.

[Item 8]

The picture coding device according to any one of items 5 to 7, wherein

the intra-color-difference prediction mode coder refers to a syntaxelement regarding an intra-color-difference prediction mode and acorresponding intra-brightness prediction mode and specifies a modenumber of the first intra-color-difference prediction mode, and

when the mode number of the first intra-color-difference prediction modeis not a value showing vertical prediction, the color-difference signalintra-predictor derives the second intra-color-difference predictionmode, on the basis of a conversion table in which the mode number isconverted to exclude a mode number showing the vertical prediction.

[Item 9]

The picture coding device according to any one of items 1 to 8, wherein

the number of candidates of the syntax element regarding theintra-color-difference prediction mode is smaller than the number ofintra-color-difference prediction modes.

[Item 10]

A picture coding method for coding information regarding anintra-prediction mode in a prediction block unit and coding picturesignals including a brightness signal and a color-difference signal in aconversion block unit using intra-prediction, comprising:

setting a prediction block of the brightness signal, coding a syntaxelement regarding the intra-prediction mode of the brightness signal, onthe basis of the intra-prediction mode of the brightness signal showingan intra-prediction method of the prediction block of the bright signal,and coding information regarding an intra-brightness prediction mode ina bitstream;

setting a prediction block of the color-difference signal, coding asyntax element regarding an intra-color-difference prediction mode ofthe color-difference signal by referring to the intra-brightnessprediction mode, on the basis of the intra-color-difference predictionmode of the color-difference signal showing an intra-prediction methodof the prediction block of the color-difference signal, and codinginformation regarding the intra-color-difference prediction mode in thebitstream;

predicting a brightness signal of a conversion block of the brightnesssignal from a surrounding brightness signal of the conversion block ofthe brightness signal, according to the intra-brightness predictionmode; and

predicting a color-difference signal of a conversion block of thecolor-difference signal from a surrounding color-difference signal ofthe conversion block of the color-difference signal, according to theintra-color-difference prediction mode.

[Item 11]

A picture coding program for coding information regarding anintra-prediction mode in a prediction block unit and coding picturesignals including a brightness signal and a color-difference signal in aconversion block unit using intra-prediction, the picture coding programcausing a computer to execute:

setting a prediction block of the brightness signal, coding a syntaxelement regarding the intra-prediction mode of the brightness signal, onthe basis of the intra-prediction mode of the brightness signal showingan intra-prediction method of the prediction block of the bright signal,and coding information regarding an intra-brightness prediction mode ina bitstream;

setting a prediction block of the color-difference signal, coding asyntax element regarding an intra-color-difference prediction mode ofthe color-difference signal by referring to the intra-brightnessprediction mode, on the basis of the intra-color-difference predictionmode of the color-difference signal showing an intra-prediction methodof the prediction block of the color-difference signal, and codinginformation regarding the intra-color-difference prediction mode in thebitstream;

predicting a brightness signal of a conversion block of the brightnesssignal from a surrounding brightness signal of the conversion block ofthe brightness signal, according to the intra-brightness predictionmode; and

predicting a color-difference signal of a conversion block of thecolor-difference signal from a surrounding color-difference signal ofthe conversion block of the color-difference signal, according to theintra-color-difference prediction mode.

[Item 12]

A picture decoding device for decoding information regarding anintra-prediction mode in a prediction block unit and decoding picturesignals including a brightness signal and a color-difference signal in aconversion block unit using intra-prediction, comprising:

an intra-brightness prediction mode decoder that decodes a syntaxelement regarding an intra-prediction mode of the brightness signal froma bitstream in which information regarding an intra-brightnessprediction mode showing an intra-prediction method of a prediction blockof the bright signal is coded and derives the intra-prediction mode ofthe brightness signal;

an intra-color-difference prediction mode decoder that decodes a syntaxelement regarding an intra-color-difference prediction mode of the colordifference signal from the bitstream in which information regarding anintra-color-difference prediction mode showing an intra-predictionmethod of a prediction block of the color-difference signal is coded andderives the intra-color-difference prediction mode by referring to theintra-brightness prediction mode;

a brightness signal intra-predictor that predicts a brightness signal ofa conversion block of the brightness signal from a surroundingbrightness signal of the conversion block of the brightness signal,according to the intra-brightness prediction mode specified for eachprediction block of the brightness signal; and

a color-difference signal intra-predictor that predicts acolor-difference signal of a conversion block of the color-differencesignal from a surrounding color-difference signal of the conversionblock of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal.

[Item 13]

The picture decoding device according to item 12, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, theintra-color-difference prediction mode decoder converts a mode number ofa first intra-color-difference prediction mode used when the aspectratios are equal to each other into a scaled mode number and derives asecond intra-color-difference prediction mode used when the aspectratios are different from each other.

[Item 14]

The picture decoding device according to item 12, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, thecolor-difference signal intra-predictor scales an angle of a predictiondirection defined by a first intra-color-difference prediction mode usedwhen the aspect ratios are equal to each other, specifies a mode numberof an intra-prediction mode of an angle close to the scaled angle, andderives a second intra-prediction mode used when the aspect ratios aredifferent from each other.

[Item 15]

The picture decoding device according to item 12, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, thecolor-difference signal intra-predictor refers to a conversion table inwhich a mode number of an intra-color-difference prediction mode usedwhen the aspect ratios are equal to each other and an angle afterscaling are associated with each other and performs the intra-predictionon the color-difference signal using a converted angle.

[Item 16]

The picture decoding device according to item 12, wherein

when aspect ratios of pixels of the brightness signal and thecolor-difference signal are different from each other, theintra-color-difference prediction mode decoder derives a secondintra-color-difference prediction mode used when the aspect ratios aredifferent from each other, on the basis of a conversion table in which avalue close to a prediction direction derived by scaling an angle of aprediction direction corresponding to a first intra-color-differenceprediction mode used when the aspect ratios are equal to each other isset.

[Item 17]

The picture decoding device according to item 16, wherein

when a value of the second intra-color-difference prediction modederived by scaling the angle of the prediction direction of the firstintra-color-difference prediction mode is beyond a range of mode numbersdefined by an intra-prediction mode, the intra-color-differenceprediction mode decoder sets the value of the secondintra-color-difference prediction mode to a value in the range of themode numbers defined by the intra-prediction mode.

[Item 18]

The picture decoding device according to item 16 or 17, wherein

the intra-color-difference prediction mode decoder refers to a syntaxelement regarding an intra-color-difference prediction mode and acorresponding intra-brightness prediction mode and specifies a modenumber of the first intra-color-difference prediction mode, and

the color-difference signal intra-predictor derives the secondintra-color-difference prediction mode, on the basis of a conversiontable in which a value obtained by scaling the mode number of the firstintra-color-difference prediction mode is set to exclude a mode numberderived when values of the intra-brightness prediction mode and thefirst intra-color-difference prediction mode are not matched with eachother.

[Item 19]

The picture decoding device according to any one of items 16 to 18,wherein

the intra-color-difference prediction mode decoder refers to a syntaxelement regarding an intra-color-difference prediction mode and acorresponding intra-brightness prediction mode and specifies a modenumber of the first intra-color-difference prediction mode, and

when the mode number of the first intra-color-difference prediction modeis not a value showing vertical prediction, the color-difference signalintra-predictor derives the second intra-color-difference predictionmode, on the basis of a conversion table in which the mode number isconverted to exclude a mode number showing the vertical prediction.

[Item 20]

The picture decoding device according to any one of items 12 to 19,wherein

the number of candidates of the syntax element regarding theintra-color-difference prediction mode is smaller than the number ofintra-color-difference prediction modes.

[Item 21]

A picture decoding method for decoding information regarding anintra-prediction mode in a prediction block unit and decoding picturesignals including a brightness signal and a color-difference signal in aconversion block unit using intra-prediction, comprising:

decoding a syntax element regarding an intra-prediction mode of thebrightness signal from a bitstream in which information regarding anintra-brightness prediction mode showing an intra-prediction method of aprediction block of the bright signal is coded and deriving theintra-prediction mode of the brightness signal;

decoding a syntax element regarding an intra-color-difference predictionmode of the color-difference signal from the bitstream in whichinformation regarding an intra-color-difference prediction mode showingan intra-prediction method of a prediction block of the color-differencesignal is coded and deriving the intra-color-difference prediction modeby referring to the intra-brightness prediction mode;

predicting a brightness signal of a conversion block of the brightnesssignal from a surrounding brightness signal of the conversion block ofthe brightness signal, according to the intra-brightness prediction modespecified for each prediction block of the brightness signal; and

predicting a color-difference signal of a conversion block of thecolor-difference signal from a surrounding color-difference signal ofthe conversion block of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal.

[Item 22]

A picture decoding program for decoding information regarding anintra-prediction mode in a prediction block unit and decoding picturesignals including a brightness signal and a color-difference signal in aconversion block unit using intra-prediction, the picture decodingprogram causing a computer to execute:

decoding a syntax element regarding an intra-prediction mode of thebrightness signal from a bitstream in which information regarding anintra-brightness prediction mode showing an intra-prediction method of aprediction block of the bright signal is coded and deriving theintra-prediction mode of the brightness signal;

decoding a syntax element regarding an intra-color-difference predictionmode of the color-difference signal from the bitstream in whichinformation regarding an intra-color-difference prediction mode showingan intra-prediction method of a prediction block of the color-differencesignal is coded and deriving the intra-color-difference prediction modeby referring to the intra-brightness prediction mode;

predicting a brightness signal of a conversion block of the brightnesssignal from a surrounding brightness signal of the conversion block ofthe brightness signal, according to the intra-brightness prediction modespecified for each prediction block of the brightness signal; and

predicting a color-difference signal of a conversion block of thecolor-difference signal from a surrounding color-difference signal ofthe conversion block of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal.

What is claimed is:
 1. A picture decoding device for decodinginformation regarding an intra-prediction mode in a prediction blockunit and decoding picture signals including a brightness signal and acolor-difference signal in a conversion block unit usingintra-prediction, comprising: an intra-brightness prediction modedecoder that decodes a first syntax element regarding anintra-prediction mode of the brightness signal from a bitstream in whichinformation regarding an intra-brightness prediction mode showing anintra-prediction method of a prediction block of a bright signal iscoded and derives the intra-brightness prediction mode from the firstsyntax element; an intra-color-difference prediction mode decoder thatdecodes a second syntax element regarding an intra-color-differenceprediction mode of the color-difference signal from the bitstream inwhich information regarding an intra-color-difference prediction modeshowing an intra-prediction method of a prediction block of thecolor-difference signal is coded and derives a firstintra-color-difference prediction mode from the second syntax elementand the intra-brightness prediction mode; a brightness signalintra-predictor that predicts a brightness signal of a conversion blockof the brightness signal from a surrounding brightness signal of theconversion block of the brightness signal, according to theintra-brightness prediction mode specified for each prediction block ofthe brightness signal; and a color-difference signal intra-predictorthat predicts a color-difference signal of a conversion block of thecolor-difference signal from a surrounding color-difference signal ofthe conversion block of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal, wherein when aspect ratios ofpixels of the brightness signal and the color-difference signal aredifferent from each other, the intra-color-difference prediction modedecoder derives a second intra-color-difference prediction mode from thefirst intra-color-difference prediction mode, based on a conversiontable in which an intra-prediction mode of a prediction direction of anangle close to an angle derived by scaling an angle of a predictiondirection corresponding to the first intra-color-difference predictionmode used when the aspect ratios are equal to each other by apredetermined scaling factor is set as the second intra-color-differenceprediction mode used when the aspect ratios are different from eachother, in the conversion table, a mode number of the secondintra-color-difference prediction mode is set, excluding a mode numbershowing vertical prediction, if a mode number of the firstintra-color-difference prediction mode is not a mode number showingvertical prediction.
 2. A picture decoding method for decodinginformation regarding an intra-prediction mode in a prediction blockunit and decoding picture signals including a brightness signal and acolor-difference signal in a conversion block unit usingintra-prediction, comprising: decoding a first syntax element regardingan intra-prediction mode of the brightness signal from a bitstream inwhich information regarding an intra-brightness prediction mode showingan intra-prediction method of a prediction block of a bright signal iscoded and derives the intra-brightness prediction mode from the firstsyntax element; decoding a second syntax element regarding anintra-color-difference prediction mode of the color-difference signalfrom the bitstream in which information regarding anintra-color-difference prediction mode showing an intra-predictionmethod of a prediction block of the color-difference signal is coded andderives a first intra-color-difference prediction mode from the secondsyntax element and the intra-brightness prediction mode; predicting abrightness signal of a conversion block of the brightness signal from asurrounding brightness signal of the conversion block of the brightnesssignal, according to an intra-brightness prediction mode specified foreach prediction block of the brightness signal; and predicting acolor-difference signal of a conversion block of the color-differencesignal from a surrounding color-difference signal of the conversionblock of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal, wherein when aspect ratios ofpixels of the brightness signal and the color-difference signal aredifferent from each other, deriving a second intra-color-differenceprediction mode from the first intra-color-difference prediction mode,based on a conversion table in which an intra-prediction mode of aprediction direction of an angle close to an angle derived by scaling anangle of a prediction direction corresponding to the firstintra-color-difference prediction mode used when the aspect ratios areequal to each other by a predetermined scaling factor is set as thesecond intra-color-difference prediction mode used when the aspectratios are different from each other, in the conversion table, a modenumber of the second intra-color-difference prediction mode is set,excluding a mode number showing vertical prediction, if a mode number ofthe first intra-color-difference prediction mode is not a mode numbershowing vertical prediction.
 3. A non-transitory computer-readablerecording medium having embodied thereon a picture decoding program fordecoding information regarding an intra-prediction mode in a predictionblock unit and decoding picture signals including a brightness signaland a color-difference signal in a conversion block unit usingintra-prediction, the picture decoding program causing a computer toexecute: decoding a first syntax element regarding an intra-predictionmode of the brightness signal from a bitstream in which informationregarding an intra-brightness prediction mode showing anintra-prediction method of a prediction block of a bright signal iscoded and derives the intra-brightness prediction mode from the firstsyntax element; decoding a second syntax element regarding anintra-color-difference prediction mode of the color-difference signalfrom the bitstream in which information regarding anintra-color-difference prediction mode showing an intra-predictionmethod of a prediction block of the color-difference signal is coded andderives a first intra-color-difference prediction mode from the secondsyntax element and the intra-brightness prediction mode; predicting abrightness signal of a conversion block of the brightness signal from asurrounding brightness signal of the conversion block of the brightnesssignal, according to an intra-brightness prediction mode specified foreach prediction block of the brightness signal; and predicting acolor-difference signal of a conversion block of the color-differencesignal from a surrounding color-difference signal of the conversionblock of the color-difference signal, according to theintra-color-difference prediction mode specified for each predictionblock of the color-difference signal, wherein when aspect ratios ofpixels of the brightness signal and the color-difference signal aredifferent from each other, deriving a second intra-color-differenceprediction mode from the first intra-color-difference prediction mode,based on a conversion table in which an intra-prediction mode of aprediction direction of an angle close to an angle derived by scaling anangle of a prediction direction corresponding to the firstintra-color-difference prediction mode used when the aspect ratios areequal to each other by a predetermined scaling factor is set as thesecond intra-color-difference prediction mode used when the aspectratios are different from each other, in the conversion table, a modenumber of the second intra-color-difference prediction mode is set,excluding a mode number showing vertical prediction, if a mode number ofthe first intra-color-difference prediction mode is not a mode numbershowing vertical prediction.